e1000: implement more efficient tx queue locking

Implement more efficient locking (avoid the lock) when checking for
a stopped queue.  Also don't wake the queue unless the threshold is
reached to avoid queue on/off thrash.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: John Ronciak <john.ronciak@intel.com>