staging: xillybus: Improved error handling

xilly_scan_idt() now returns an error status code, rather than being a void
function and hint the status through an entry in a data structure.

Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>