iwlwifi: mvm: fix Tx Power firmware API

The firmware doesn't relate the scan to a vif. The scan is
run by a separate entity called auxiliary MAC (aka AUX MAC).
This AUX MAC needs to get Tx power limitations that are
not applied on a specific vif, but on the device as a whole.

This can be implemented by using the minimum of all the
values set by the user for all the MACs. But then we need
to ignore the limitations that come from the AP or
regulatory for a specific vif: a specific vif might have
regulatory limitations because of the channel is works on.
This limit is irrelevant for the AUX MAC.
Use the new API from mac80211: the user_power_level in
bss_conf to achieve this.

Firmware -13.ucode has already moved to this API.

Change-Id: Ifba83660f378e91b93bd46d29fe8ba35a7c168a4
Signed-off-by: Avri Altman <avri.altman@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>