ath9k_hw: clean up tx power handling

The code for handling various restrictions concerning regulatory limits,
antenna gain, etc. is very convoluted and duplicated across various
EEPROM parsing implementations, making it hard to review.

This patch partially cleans up the mess by unifying regulatory limit
handling in one function and simplifying handling of antenna gain.
It also removes unused transmit power scaling arrays from the EEPROM code,
which belonged to an unimplemented API that isn't supposed to be in
the driver anyway.

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