iwlwifi: mvm: don't use reduced Tx power when not applicable

When we have only one antenna for BT and WiFi, reduced Tx
power is irrelevant.
Also, in loose scheme, we should not use reduced Tx power
nor set the control mask to Tx power.

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