ar9170: implement frequency calibration for one-stage/openfw

This patch ports some code from the vendor driver, which is
supposed to upload the right calibration values for the
chosen frequency.

In theory, this should give a better range and throughput
for all users with the open, or one-stage firmware.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>