HID: wacom: break out parsing of device and registering of input

Simplifies the .probe() and will allow to reuse this path in the future.
Few things are reshuffled in .probe():
- init wacom struct earlier
- then retrieve the report descriptor
- then parse it and allocate/register inputs.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>