carl9170: set beacon xmit power to the max

Harshal Chhaya discovered during network tests, that
several of his clients dropped-off the network. The
captured packets shows that the beacons sent by the
AP are at a much lower power than the other data
packets.

The reason for this mishap: The driver never updated
the beacon phy register, so all beacons were always
sent at the lowest power.

Reference: http://marc.info/?l=linux-wireless&m=131067225105801
Reported-by: Harshal Chhaya <harshal@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>