b43legacy: Load firmware from work queue instead of from probe routine

Recent changes in udev are causing problems for drivers that load firmware
from the probe routine. As b43legacy has such a structure, it must be changed.
As this driver loads 3 or 4 firmware files, changing to the asynchronous routine
request_firmware_nowait() would be complicated. In this implementation, the probe
routine starts a work queue that calls the firmware loading routines.

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