brcmfmac: kill URB when request timed out

Kill the submitted URB in brcmf_usb_dl_cmd if the request timed out. This
assures the URB is never submitted twice. It also prevents a possible
use-after-free of the URB transfer buffer if a timeout occurs.

Signed-off-by: Mathy Vanhoef <vanhoefm@gmail.com>
Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>