cnss2: Return proper value when sending AOP msg

Positive return value from mbox_send_message() should be
treated as success, which indicate the index of
free mbox buffer obtained.

Change-Id: I59b47a3a711468ef233b43c1191217f1f6911c1f
CRs-Fixed: 3878630

cnss2/power.c

@@ -1426,6 +1426,8 @@ cnss_mbox_send_msg(struct cnss_plat_data *plat_priv, char *mbox_msg)
 	ret = mbox_send_message(plat_priv->mbox_chan, &pkt);
 	if (ret < 0)
 		cnss_pr_err("Failed to send AOP mbox msg: %s\n", mbox_msg);
+	else
+		ret = 0;
 
 	return ret;
 }