igb: change descriptor control thresholds

This change simplifies the code by setting RX_PTHRESH to 8 for
all devices, as it was unlikely that there was any advantage to
set it at 16 for earlier cards. Additionally TX_WTHRESH is
set to 1 for the 82576 NIC to improve performance by enabling
a minimal amount of write combining when writing back descriptors.

Signed-off-by: Nicholas Nunley <nicholasx.d.nunley@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>