rt2x00: Use unlikely for unexpected error condition in rt2x00_mac_tx

rt2x00queue_write_tx_frame is unlikely to fail. Tell the compiler.

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>