wl1271: Increase TX power value

Currently, to avoid distortions, the TX power level has been hardcoded
to a low value. The value is slightly too low for good functionality, so
we increase it from 7dB to 12dB.

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