ath9k_hw: make support for PC-OEM cards optional

The initvals use up quite a bit of space, and PC-OEM support is
typically not needed on embedded systems

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

ath9k_hw: Fix RTT calibration

This patch fixes multiple issues with the current RTT
implementation in ath9k.

* The data that is obtained from the RTT interface registers
are stored in 31:5 - mask out the extra bits when reading them.

* A history buffer is maintained which is not needed at all.
Remove this array and just store the baseband data for each
chain (or bank).

* A 'num_readings' variable was being used to handle the
last entry. But it was being used in an improper manner, with
the result that the RTT values were never being written
to the RTT Interface registers. Fix this by using a simple
flag.

* Stop baseband operations before programming the calibration values
to the HW.

* Do not restore RX gain settings as part of RTT.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

ath9k_hw: Add radio retention support for AR9480

Supported calibrations of radio retention table (RTT) are
- DC offset
- Filter
- Peak detect

Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>