ath10k: use bss_info as txpower source

This simply changes the source for txpower setup.
It does not make ath10k use different txpower
values for different vifs.

This will make it easier to implement chanctx in
ath10k in the future.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>