e1000e: drop stats lock

the stats lock is left over from e1000, e1000e no longer
has the adjust tbi stats function that required the addition
of the stats lock to begin with.

adding a mutex to acquire_swflag helped catch this one too.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>