Check for if the usb_alloc_urb() failed.

Fixes: 68d57a07 ("wireless: add plfxlc driver for pureLiFi X, XL, XC devices")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/e8d4a19a-f251-4101-a89b-607345e938cb@moroto.mountain