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>