b43: Fix PPC crash in rfkill polling on unload

In Bugzilla No. 14181, a PowerMac G4 crashes on ifdown or
module unload because the rfkill polling has not been stopped.
For the x86 architectures, the attempt to reach a now unmapped
register is not fatal as it is on PPC.

(Includes "b43: Fix locking problem when stopping rfkill polling". -- JWL)

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>