ibmvnic: Account for VLAN header length in TX buffers

The extra four bytes of a VLAN packet was throwing off
TX buffer entry values used by the driver. Account for those
bytes when in buffer size and buffer entry calculations

Signed-off-by: Thomas Falcon <tlfalcon@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>