ALSA: usx2y: remove an unneeded check

The test here is always true because S[i].urb is an array not a pointer.
Also it's bogus because the intent was to test:
	if (S->urb[i]) {
instead of:
	if (S[i].urb) {

Anyway, usb_kill_urb() and usb_free_urb() accept NULL pointers so we can
just remove this.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>