wl1271: add workaround to avoid distortion due to excessive tx power

We still don't have proper calibration for our devices, so we are using way
too much power for TX, which causes a lot of distortion.  This hack hardcodes
the txpower to 7dBm.

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>