net: lwip: Fix PBUF_POOL_BUFSIZE when PROT_TCP_LWIP is disabled

net: lwip: Fix PBUF_POOL_BUFSIZE when PROT_TCP_LWIP is disabled

The PBUF_POOL_BUFSIZE ends up being only 592 bytes, instead of 1514,
when PROT_TCP_LWIP Kconfig option is disabled. This results in a full
Ethernet frame requiring three PBUFs instead of just one.

This happens because the PBUF_POOL_BUFSIZE constant depends on the value
of a TCP_MSS constant, something that defaults to 536 when PROT_TCP_LWIP
is disabled.

  PBUF_POOL_BUFSIZE = LWIP_MEM_ALIGN_SIZE(TCP_MSS + 40 + PBUF_LINK_HLEN)

Ensure that a full Ethernet frame fits inside a single PBUF by moving
the define of TCP_MSS outside the PROT_TCP_LWIP ifdef block.

Fixes: 1c41a7afaa15 ("net: lwip: build lwIP")
Acked-by: Jerome Forissier <jerome.forissier@arm.com>
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>