b44: Disable device on shutdown

Disable the SSB core on device shutdown.
This has two advantages:
1) A clean device shutdown is always desired here, because we disable
  the device's global crystal in the next statement.
2) This fixes a bug where the device will come up with the enable-bit
  set on the next initialization (without a reboot inbetween).
  This causes breakage on the second initialization due to code that
  checks this bit (ssb_device_is_enabled() checks).

Reported-by: Pantelis Koukousoulas <pktoss@gmail.com>
Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: David S. Miller <davem@davemloft.net>