mailbox: pcc: Return early if no GAS register from pcc_mbox_cmd_complete_check

pcc_mbox_cmd_complete_check() accesses pchan->cmd_complete.gas to check
command completion status. Even if GAS is NULL, pcc_chan_reg_read() gets
called which returns success doing nothing and then we return.

Add an early return if pchan->cmd_complete.gas == NULL before performing
any operations.

Acked-by: Huisong Li <lihuisong@huawei.com>
Tested-by: Huisong Li <lihuisong@huawei.com>
Tested-by: Adam Young <admiyo@os.amperecomputing.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>

Author: sudeep-holla

drivers/mailbox/pcc.c

@@ -245,13 +245,13 @@ static bool pcc_mbox_cmd_complete_check(struct pcc_chan_info *pchan)
 	u64 val;
 	int ret;
 
+	if (!pchan->cmd_complete.gas)
+		return true;
+
 	ret = pcc_chan_reg_read(&pchan->cmd_complete, &val);
 	if (ret)
 		return false;
 
-	if (!pchan->cmd_complete.gas)
-		return true;
-
 	/*
 	 * Judge if the channel respond the interrupt based on the value of
 	 * command complete.