This change makes it possible to limit the number of descriptors down to 48
per ring.  The reason for this change is to address a variation on hardware
errata 10 for 82546GB in which descriptors will be lost if more than 32
descriptors are fetched and the PCI-X MRBC is 512.
Signed-off-by: NAlexander Duyck <alexander.h.duyck@intel.com>
Tested-by: NEmil Tantilov <emil.s.tantilov@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>