iwlwifi: mvm: allow preventing dummy notifications

The firwmare now allows the driver to disable dummy
notifications. These notifications sent by the firmware
are an overhead for slow buses. They are still useful for
fast buses.
Add a hardware switch to prevent these notifications only
on devices that work on slow buses.

Signed-off-by: Eran <eran.harary@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>