gianfar: Fix frame size calculation when hardware VLAN acceleration is on

In gfar_change_mtu(), the frame size needs to be increased to account
for the extra 4 bytes VLAN adds to the ethernet header.  However,
it was being increased by the length of the whole header (18 bytes),
which is wrong.

Signed-off-by: Dai Haruki <dai.haruki@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>