ath10k: 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 no of tx chain masks,
for QCA9984 allowed tx power range values 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.

Tested Hardware : QCA9984
Tested Firmware : 10.4-3.9.0.2-00046

Co-developed-by: Balaji Pothunoori <bpothuno@codeaurora.org>
Signed-off-by: Ashok Raj Nagarajan <arnagara@codeaurora.org>
Signed-off-by: Balaji Pothunoori <bpothuno@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>