rt2x00: Don't kill guardian_urb when it wasn't created

This fixes a "BUG: unable to handle kernel paging request"
bug in rt73usb which was caused by killing the guardian_urb
while it had never been allocated for rt73usb.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>