ath,ar9170: implemented conformance test limit calc. for tx power

ath,ar9170: implemented conformance test limit calc. for tx power

apply the conformance test limits (CTL) stored in the eeprom upon
the values calculated for the tx power (ar->power_*).

This is based on the implementation in the vendor driver
(hal/hpmain.c, line 3700 ff.) with one difference:
If any ctl mode isn't found in the eeprom, we fall back to the "lower",
legacy modes (5GHT20,11A or 2GHT20,11G,11B). Otus only did 5GHT20->11A.

Currently CTL are applied for the FCC group only.

Signed-off-by: Joerg Albert <jal2@gmx.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>