rt2x00: Don't leak mem in error path of rt2x00lib_request_firmware()

We need to release_firmware() in order not to leak memory.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Pekka Enberg <penberg@kernel.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>