cifs: cifs_convert_address() returns zero on error

The cifs_convert_address() returns zero on error but this caller is
testing for negative returns.

Btw. "i" is unsigned here, so it's never negative.

Reviewed-by: Jeff Layton <jlayton@samba.org>
Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>