rt2x00: Use unlikely for error case in rt2x00queue_write_tx_frame

This is an error condition that is not supposed to happen. Hence, it is
safe to add unlikely to this check.

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