wacom: Keep GsdWacomDeviceType on the stack in device_added_cb()

So gsd_wacom_device_get_device_type() isn't called multiple times.
wacom: Check for calibration state on startup/device-added

So the user is notified when plugging a brand new tablet, or a
previously known one that's maybe miscalibrated on the current
set-up.

https://bugzilla.gnome.org/show_bug.cgi?id=700461