net: lan78xx: Reduce s/w csum check on VLANs

With HW_VLAN_CTAG_RX enabled we don't observe the checksum
issue, so amend the workaround to only drop back to s/w
checksums if VLAN offload is disabled.

See #2458.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>