w1: silence an uninitialized variable warning

If kstrtoint() returns -ERANGE then "tmp" is uninitialized.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Evgeniy Polaykov <zbr@ioremap.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>