ixgbe: A fix to VF TX rate limit

There is a need to configure MMW_SIZE in register RTTBCNRM with a correct
value (0x4 for non jumbo frames and 0x14 for jumbo frames support).
For 82599 the value is 0x4 and for X540 the value is 0x14.

Signed-off-by: Lior Levy <lior.levy@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>