b43: Fix possible MMIO access while device is down

This fixes a possible MMIO access while the device is still down
from a suspend cycle. MMIO accesses with the device powered down
may cause crashes on certain devices.

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