HID: wacom: split out input allocation and registration

If the input can be created earlier during probe, we can already populate
them while reading the report descriptor. This way, we can rely on the
hid subsystem directly for tablets which already provide a meaningful
report descriptor (like ISDv4-5).

This patch only splits the allocation and registration, but do not
change where we allocate the input. This will come in a later patch.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Jason Gerecke <killertofu@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>