igb: update handling of RCTL for smaller buffer sizes

As we begin to move into virtualization the use of a global value for
buffer sizes becomes undesirable.  This change makes it so only 256 and 512
byte buffers require use of the RCTL register.

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>