igb: remove unneeded bit refrence when enabling jumbo frames

There is a reference to a Buffer Size extention bit that is unneded by
82575/82576 hardware.  Since it is not needed it should be removed from the
code.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>