igb: increase minimum rx buffer size to 1K

This update increases the minimum rx buffer size to 1K.  The reason for this
change is to support SR-IOV and avoid any conflicts with the rings being able
to set their own MTU sizes.

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>