staging: xillybus: fix error return code in xilly_probe()

Fix to return negative error code -EIO from the error handling
case instead of 0.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Reviewed-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>