staging: xillybus: Fixes related to "rc" variable

"rc" is often used to hold the return value of a function call. This patch
removes unnecessary assignments to this variable, and makes a few related
execution flow improvements.

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>