mailbox: mpfs: fix copy and paste bug in probe

This code accidentally checks ->ctrl_base instead of ->mbox_base so the
error handling can never be triggered.

Fixes: a4123ff ("mailbox: mpfs: support new, syscon based, devicetree configuration")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>

Author: Dan Carpenter

drivers/mailbox/mailbox-mpfs.c

@@ -251,7 +251,7 @@ static inline int mpfs_mbox_syscon_probe(struct mpfs_mbox *mbox, struct platform
 		return PTR_ERR(mbox->sysreg_scb);
 
 	mbox->mbox_base = devm_platform_ioremap_resource(pdev, 0);
-	if (IS_ERR(mbox->ctrl_base))
+	if (IS_ERR(mbox->mbox_base))
 		return PTR_ERR(mbox->mbox_base);
 
 	return 0;