iwlwifi: mvm: implement reduced Tx power

This allows to have better wifi TPT when BT is active under
good RSSI conditions.
Wifi will have better chance to send Acks and Cts even if BT
is active.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>