ixgb: remove lock access in the fast path

This mimics a change made in the e1000 driver that imitates a slick
tg3 way of avoiding grabbing the lock around restarting the tx queue.

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>