iwlwifi: use __get_str in tracing

Instead of using (char *)__get_dynamic_array use
__get_str. The latter is actually a macro that
expands to the former in the code, but trace-cmd
in userspace can parse __get_str only.
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

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

@@ -298,7 +298,7 @@ TRACE_EVENT(iwlwifi_dbg,
 				       MAX_MSG_LEN, vaf->fmt,
 				       *vaf->va) >= MAX_MSG_LEN);
 	),
-	TP_printk("%s", (char *)__get_dynamic_array(msg))
+	TP_printk("%s", __get_str(msg))
 );
 
 #undef TRACE_SYSTEM