tg3: Drop non-VLAN rx pkts larger than the MTU

The hardware's MTU is configured so that it will accept packets whose
VLAN tag is left inline.  This means that the hardware will also accept
other packets larger than the desired MTU size.  This patch checks for
and discards packets that are larger than the requested MTU but are not
VLAN tagged frames.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>