b43: Remove the PHY spinlock

This fixes a sparse warning about weird locking.
The spinlock is not needed, so simply remove it.
This also adds some sanity checks to the PHY and radio locking
to protect against recursive locking.

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