e1000e: fix logical error in flush_desc_rings

The condition under which the flush should occur was reversed.  The fix
should be applied before any HW reset (unless followed by bus reset)
and before any power state transition from D0.

If E1000_FEXTNVM7_NEED_DESCRING_FLUSH bit is set in FEXTNVM7 and TDLEN > 0
the Tx ring should be flushed. (fixes ~95% of the hang states).
If the E1000_FEXTNVM7_NEED_DESCRING_FLUSH did not clear, we should also
flush the RX ring. Bug was caught by Alexander Duyck during a code review
when examining this fix.

Signed-off-by: Yanir Lubetkin <yanirx.lubetkin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>