e1000e: Set HTHRESH when PTHRESH is used

According to section 12.0.3.4.13 "Receive Descriptor Control - RXDCTL"
of the Intel® 82579 Gigabit Ethernet PHY Datasheet v2.1:

    "HTHRESH should be given a non zero value when ever PTHRESH is
     used."

In RXDCTL(0), PTHRESH lives at bits 5:0, and HTHREST lives at bits 13:8.
Set only bit 8 of HTHREST as is done in e1000_flush_rx_ring(). Found by
inspection.
Signed-off-by: Matt Turner <matt.turner@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/e1000e/netdev.c

@@ -3303,7 +3303,7 @@ static void e1000_configure_rx(struct e1000_adapter *adapter)
 		if (adapter->flags & FLAG_IS_ICH) {
 			u32 rxdctl = er32(RXDCTL(0));
 
-			ew32(RXDCTL(0), rxdctl | 0x3);
+			ew32(RXDCTL(0), rxdctl | 0x3 | BIT(8));
 		}
 
 		pm_qos_update_request(&adapter->pm_qos_req, lat);