b43: Fix some MAC locking

This fixes some locking w.r.t. the lower MAC (firmware).
It also removes a lot of ancient IRQ-locking that's not needed anymore.
We simply suspend the MAC. That's easier and causes less trouble.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>