staging: xillybus: Return -EIO if *_iomap fails, instead of 0

This patch replicates the correction made by Wei Yongjun on a second
occurrence of the same bug.

The first correction was in commit 8eec4555.

Bug fixed: The error code was not set, so the error condition wasn't reflected
in the return value.
Reported-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/xillybus/xillybus_of.c

@@ -131,10 +131,10 @@ static int xilly_drv_probe(struct platform_device *op)
 	}
 
 	endpoint->registers = of_iomap(dev->of_node, 0);
-
 	if (!endpoint->registers) {
 		dev_err(endpoint->dev,
 			"Failed to map I/O memory. Aborting.\n");
+		rc = -EIO;
 		goto failed_iomap0;
 	}