net: guard SYS_RX_ETH_BUFFER with NET

net: guard SYS_RX_ETH_BUFFER with NET

SYS_RX_ETH_BUFFER represents the number of Ethernet receive packet
buffers. It therefore doesn't make sense it's reachable if NET isn't
enabled.

Direct users of SYS_RX_ETH_BUFFER are:
- drivers/net/rtl8169.c, only compiled if CONFIG_RTL8169=y, depends on
  CONFIG_NETDEVICES=y, depends on CONFIG_NET=y,
- drivers/net/fsl_enetc.h, via ENETC_BD_CNT, included in
    drivers/net/{fsl_enetc.c,fsl_enetc_mdio.c,mscc_eswitch/felix_switch.c}
  First two only compiled if CONFIG_FSL_ENETC=y, latter with
  CONFIG_MSCC_FELIX_SWITCH=y. Both symbols depends on
  CONFIG_NETDEVICES=y, depends on CONFIG_NET=y.
- include/net-common.h via PKTBUFSRX,

Indirect users via PKTBUFSRX:
- arch/sandbox/include/asm/eth.h
  - according to ./tools/qconfig.py -l -f CONFIG_SANDBOX CONFIG_NO_NET,
    all sandbox defconfigs have network enabled so ignore this for now,
- drivers/dma/ti/k3-udma.c
  - sets UDMA_RX_DESC_NUM to that if defined, else 4. PKTBUFSRX is
    CONFIG_SYS_RX_ETH_BUFFER which defaults to 4. According to
    ./tools/qconfig.py -l -f CONFIG_TI_K3_NAVSS_UDMA '~CONFIG_SYS_RX_ETH_BUFFER=4'
    no defconfig enabling this DMA driver sets CONFIG_SYS_RX_ETH_BUFFER
    to anything but the default of 4, so regardless of NET being built
    UDMA_RX_DESC_NUM will always be 4 with current defconfigs.
- drivers/net/{airoha_eth.c,bcm6348-eth.c,bcm6368-eth.c,cortina_ni.c,
	dc2114x.c,eepro100.c,essedma.c,ethoc.c,ftgmac100.c,ftmac100.c,
	hifemac.c,mcffec.c,mpc8xx_fec.c,pic32_eth.c,sandbox.c,sni_ave.c,
	sni_netsec.c,ti/am65-cpsw-nuss.c,ti/cpsw.c,ti/icssg_prueth.c,
	tsec.c} all depends on CONFIG_NETDEVICES=y, depends on
  CONFIG_NET=y,
- net/lwip/net-lwip.c, only compiled if CONFIG_NET_LWIP=y, depends on
  CONFIG_NET=y,
- net/{net.c,tcp.c}, only compiled if CONFIG_NET_LEGACY=y, depends on
  CONFIG_NET=y,
- net/net-common.c, only compiled if CONFIG_NET=y,
- test/cmd/wget.c, only compiled if CONFIG_NET_LEGACY=y, depends on
  CONFIG_NET=y,
- test/image/spl_load_net.c, only compiled if CONFIG_SPL_UT_LOAD_NET=y,
  depends on CONFIG_SPL_ETH=y, depends on CONFIG_SPL_NET=y, depends on
  CONFIG_NET_LEGACY=y, depends on CONFIG_NET=y,

Indirect users via net_rx_packets[PKTBUFSRX]. This array is only
externally defined in net/net-common.c which is only compiled if
CONFIG_NET=y.

Users of net_rx_packets are:
- drivers/net/{airoha_eth.c,bcm6348-eth.c,bcm6368-eth.c,cortina_ni.c,
	dc2114x.c,dm9000x.c,essedma.c,ethoc.c,fsl_enetc.c,ftgmac100.c,
	ftmac100.c,hifemac.c,ks8851_mll.c,macb.c,mcffec.c,mpc8xx_fec.c,
        mscc_eswitch/jr2_switch.c,mscc_eswitch/luton_switch.c,
        mscc_eswitch/ocelot_switch.c,mscc_eswitch/serval_switch.c,
        mscc_eswitch/servalt_switch.c,pic32_eth.c,sandbox-raw.c,
	sandbox.c,smc911x.c,sni_ave.c,sni_netsec.c,ti/am65-cpsw-nuss.c,
	ti/cpsw.c,ti/icssg_prueth.c,tsec.c,xilinx_axi_mrmac.c} all
	depends on CONFIG_NETDEVICES=y, depends on CONFIG_NET=y,
- drivers/usb/gadget/ether.c only built if CONFIG_$(PHASE_)USB_ETHER=y,
  depends on CONFIG_NET=y/CONFIG_SPL_NET=y,
- net/lwip/net-lwip.c only compiled if CONFIG_NET_LWIP=y, depends on
  CONFIG_NET=y,
- net/net.c, only compiled if CONFIG_NET_LEGACY=y, depends on
  CONFIG_NET=y,
- net/net-common.c, only compiled if CONFIG_NET=y,

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>