igb: A fix to VF TX rate limit

There is a need to configure MMW_SIZE in register RTTBCNRM with a correct
value. For 82576 device, the value should be 0x14.

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