mac80211: ath10k: adjust tx power reduction for US regulatory domain

mac80211: ath10k: adjust tx power reduction for US regulatory domain

FCC allows maximum antenna gain of 6 dBi. 15.247(b)(4):

> (4) The conducted output power limit
> specified in paragraph (b) of this section
> is based on the use of antennas
> with directional gains that do not exceed
> 6 dBi. Except as shown in paragraph
> (c) of this section, if transmitting
> antennas of directional gain greater
> than 6 dBi are used, the conducted
> output power from the intentional radiator
> shall be reduced below the stated
> values in paragraphs (b)(1), (b)(2),
> and (b)(3) of this section, as appropriate,
> by the amount in dB that the
> directional gain of the antenna exceeds
> 6 dBi.

https://www.gpo.gov/fdsys/pkg/CFR-2013-title47-vol1/pdf/CFR-2013-title47-vol1-sec15-247.pdf

Signed-off-by: Sven Eckelmann <sven@narfation.org>