e1000e: debug contention on NVM SWFLAG

This patch adds a mutex to the e1000e driver that would help
catch any collisions of two e1000e threads accessing hardware
at the same time.

description and patch updated by Jesse

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