ath9k: make PA linearization optional, disabled by default and fix checks

Some checks for PA linearization support checked ATH9K_HW_CAP_PAPRD and some
used the EEPROM ops, leading to issues in tx power handling, since those
two can be out of sync.

Disable the feature by default, since it has been reported that it can
cause damage to the rx path under some circumstances. It can now be enabled
for testing via debugfs.

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