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: a4123ffab9ec ("mailbox: mpfs: support new, syscon based, devicetree configuration")
Signed-off-by: Dan Carpenter <[e-mail]>
Reviewed-by: Conor Dooley <[e-mail]>
Signed-off-by: Jassi Brar <[e-mail]>

drivers/mailbox/mailbox-mpfs.c

···
 		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;