b43legacy: Fix unload oops if firmware is not available

The asyncronous firmware load uses a completion struct to hold firmware
processing until the user-space routines are up and running. There is.
however, a problem in that the waiter is nevered canceled during teardown.
As a result, unloading the driver when firmware is not available causes an oops.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Stable <stable@vger.kernel.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>