net/smsc911x.c: Set the VLAN1 register to fix VLAN MTU problem

The smsc911x driver would drop frames longer than 1518 bytes, which is a
problem for networks with VLAN tagging. The VLAN1 tag register is used
to increase the legal frame size to 1522 when a VLAN tag is identified.

Signed-off-by: Göran Weinholt <weinholt@csbnet.se>
Signed-off-by: David S. Miller <davem@davemloft.net>