this patch fixes kernel Oops on "rmmod b43" if firmware was not loaded:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000088
IP: [<ffffffff8104e988>] drain_workqueue+0x25/0x142
PGD 153ac6067 PUD 153b82067 PMD 0
Oops: 0000 [#1] SMP
Signed-off-by: NOleksij Rempel <bug-track@fisher-privat.net>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>