carl9170: allow users to lower output power level

This patch implements a simple way of reducing the
output power of the device by a configurable upper
limit.

Requested-by: Harshal Chhaya <harshal@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>