xen/gntdev: remove redundant non-zero check on ret

The non-zero check on ret is always going to be false because
ret was initialized as zero and the only place it is set to
non-zero contains a return path before the non-zero check. Hence
the check is redundant and can be removed.

[ jgross@suse.com: limit scope of ret ]

Addresses-Coverity: ("Logically dead code")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>