net: usb: lan78xx: disable VLAN filter in promiscuous mode

net: usb: lan78xx: disable VLAN filter in promiscuous mode

The hardware VLAN filter (RFE_CTL_VLAN_FILTER_) drops VLAN-tagged frames
whose VID has not been registered via lan78xx_vlan_rx_add_vid(). It is
left enabled in promiscuous mode, so packet capture (e.g. tcpdump or
Wireshark) does not see tagged frames for unregistered VIDs.

Clear the filter while the interface is promiscuous and restore it from
NETIF_F_HW_VLAN_CTAG_FILTER otherwise. Enforce the same condition in
lan78xx_set_features() so netdev_update_features() cannot re-enable the
filter while promiscuous.

Fixes: 55d7de9de6c3 ("Microchip's LAN7800 family USB 2/3 to 10/100/1000 Ethernet device driver")
Signed-off-by: Enrico Pozzobon <enrico.pozzobon@dissecto.com>
Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>
Link: https://patch.msgid.link/20260701-lan78xx-vlan-promisc-v3-1-232266d32743@dissecto.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>