iwlwifi: use __get_str in tracing

__get_str() is identical to (char *)__get_dynamic_array()
that is in the code now, substitute __get_str to make the
code more readable.
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

drivers/net/wireless/iwlwifi/iwl-devtrace.h

@@ -189,7 +189,7 @@ DECLARE_EVENT_CLASS(iwlwifi_msg_event,
 				       MAX_MSG_LEN, vaf->fmt,
 				       *vaf->va) >= MAX_MSG_LEN);
 	),
-	TP_printk("%s", (char *)__get_dynamic_array(msg))
+	TP_printk("%s", __get_str(msg))
 );
 
 DEFINE_EVENT(iwlwifi_msg_event, iwlwifi_err,