w1-gpio: fix unused variable warning

Commit 8a1861d997 ("w1-gpio: Simplify & get rid of defines") removed the
compile guards from the device-tree id table, thereby generating a
warning when building without device-tree support.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>