ucc_geth: Add 16 bytes to max TX frame for VLANs

Creating a VLAN interface on top of ucc_geth adds 4 bytes
to the frame and the HW controller is not prepared to
TX a frame bigger than 1518 bytes which is 4 bytes too
small for a full VLAN frame. Add 16 bytes which will handle
the a simple VLAN and leaves 12 bytes for future expansion.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: David S. Miller <davem@davemloft.net>