net: dsa: lantiq_gswip: fix maximum frame length

Currently, outgoing packets larger than 1496 bytes are dropped when
tagged VLAN is used on a switch port.

Add the frame check sequence length to the value of the register
GSWIP_MAC_FLEN to fix this. This matches the lantiq_ppa vendor driver,
which uses a value consisting of 1518 bytes for the MAC frame, plus the
lengths of special tag and VLAN tags.

Fixes: 14fceff4 ("net: dsa: Add Lantiq / Intel DSA driver for vrx200")
Cc: stable@vger.kernel.org
Signed-off-by: Jan Hoffmann <jan@3e8.eu>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>