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 8eec4555511bfa1f8a2e0a2de45c988fd30c3efb.

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>