ath9k: interpret requested txpower in EIRP domain

Tx power limitations at upper layers are interpreted in
the EIRP domain. When the user requests a given maximum
txpower, e.g. with: 'iw phy0 set txpower fixed 1500',
he expects the EIRP to be at or below 15dBm.

In ath9k_hw_apply_txpower(), the interpretation is
different: the antenna-gain is capped against the
current txpower limit in the regulatory, but not
against the user set value. It ensures that the
resulting EIRP is below the limit defined by the
active countrycode, but not below the value the
user requested.

In a scenario like e.g.
 a) antenna_gain=6
 b) countrycode limits to eirp=18
 c) user set txpower=15
this will cause a setting for AR_PHY_POWER_TX_RATE
regs resulting in an EIRP > 15.

This patch ensures that antenna-gain is considered
whenever the txpower limit is adjusted and with that
the user set limits are kept.

Signed-off-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>