gianfar: Fix VLAN HW feature related frame/buffer size calculation.

Optimize the VLAN checking logic as well.

Signed-off-by: Dai Haruki <dai.haruki@freescale.com>
Acked-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>