ath9k_hw: enable PA linearization

ath9k_hw: enable PA linearization

This feature had been disabled in ath9k because the code to support
it was incomplete, but now the code is in sync with the internal QCA
codebase, so it's time to enable it.

On many newer devices, the calibration is assumed to be done with PA
linearization enabled.

Tests with a particular AR933x device showed that the signal emitted
at full power was highly distorted and unreliable with PA linearization
disabled. With this patch, the signal becomes clear and stability
is improved.

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