iwlwifi: mvm: rs: avoid ss_force from being reset after tx idle

ss_force is a debugging option to force a certain single stream
tx mode. It's not useful if it gets reset after tx idle. Fix that.
While at it also make sure any code touching ss_force will only
get compiled if debugfs support is configured.

Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>