iwlwifi: mvm: Introduce an API to set STA_FLG_DISABLE_TX flag

Introduce new station flag STA_FLG_DISABLE_TX, which is modified with ADD_STA
command. This flag, when set, disables tx to the STA.
Provide an API (iwl_mvm_sta_modify_disable_tx) to modify this flag, which
should be used in channel switch and immediate quiet flows.

Signed-off-by: Andrei Otcheretianski <andrei.otcheretianski@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>