input-settings: treat a serial of 1 as serial of 0

The Wacom Xorg driver assigns a serial number of 1 for any pad that doesn't
have a serial. libinput assigns 0. Just treat 1 as 0 here, there are no pens
with a real serial 1 anyway.

Fixes GNOME/mutter#414
2 jobs for <span class="ref-name">wip/wacom-generic-settings</span> in 5 minutes and 56 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #169836
build-mutter

00:02:07

 
  Test
passed #169837
test-mutter

00:03:48