ath9k: fix power limits on init

The tx power applied by set_txpower is limited by the CTL (conformance
test limit) entries in the EEPROM. These can change based on the user
configured regulatory domain.
Depending on the EEPROM data this can cause the tx power to become too
limited, if the original regdomain CTLs impose lowr limits than the CTLs
of the user configured regdomain.

To fix this issue, set the initial channel limits without any CTL
restrictions and only apply the CTL at run time when setting the channel
and the real tx power.

Signed-off-by: Felix Fietkau <nbd@nbd.name>