iwlwifi: mvm: add option that allows a vif to disable PS

We need to disable PS when a monitor vif is active or, in the future,
when a channel switch is happening.  Add a boolean to mvmvif that
allows PS to be disabled generically.  Additionally, make the monitor
interface use this new flag when it gets activated.

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>