iwlwifi: mvm: re-enable ps when monitor interfaces are removed

If a monitor interface is added and then removed, we don't reset the
mvm->ps_disabled flag, so we never re-enable power saving.  Fix that
and rearrange the code a bit.

Additionally, fix a small indentation mistake in the
iwl_mvm_power_set_pm() function declaration.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>