wl1271: limit TX power to 25dBm for every channel

The wl1271 firmware supports maximun 25.5dBm, so the driver was returning
-EINVALID to anything above that.  This patch uses the channel max_power
option to limit the TX power to 25dBm.

Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Reviewed-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>