cxgb3: FIx VLAN over Jumbo frames

The mac is expected to auto-inflate the Maximum Frame size for VLAN
tagged frames. It however does not work with jumbo frames.
Work around the bug adding 4 to the Maximum Frame for MTUs
greater than 1536.

Signed-off-by: Divy Le Ray <divy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>