rt2x00: fix cancelling uninitialized work

{rx,tx}done_work's are only initialized for usb devices.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Cc: stable@kernel.org
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>