iwlwifi: mvm: remove unused arguments

The str/len arguments to iwl_fw_dbg_trigger_simple_stop() aren't used,
and for a simple trigger don't really need to be used as the trigger
code itself encodes the reason, so remove them.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>