m68knommu: fix FEC driver locking

It's easy: grab locks before talking to hardware and realease
them afterwards. The one big lock has been splitted into a hw_lock
and mii_lock.

Signed-off-by: Sebastian Siewior <bigeasy@linutronix.de>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>