b43: Suspend MAC while killing the radio

We should suspend the MAC, before we kill the radio. This gives
the MAC a chance to leave any TX/RX state and it avoids races on
the PHY/RADIO registers.

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