ath11k: add support for controlling tx power to a station

This patch will add the support to control the transmit power
for traffic to a station associated with the AP.

Underlying firmware will enforce that the maximum tx power will
be based on the regulatory requirements. If the user given
transmit power is greater than the allowed tx power in the given
channel, then the firmware will use the maximum tx power in the
same channel.

Max and Min tx power values will depends on number of tx chain
masks. The allowed tx power range values are from 6 to 23.

When 0 is sent to the firmware as tx power, it will revert to
the default tx power for the station.

Signed-off-by: Maharaja Kennadyrajan <mkenna@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>