HID: hid-multitouch: Simplify setup and frame synchronization

With the input_configured() callback in place, the setup and frame
synchronization can be simplified. The input device initialization is
moved to mt_input_configured(), to make sure the full HID report has been
seen.

Reviewed-and-tested-by: Benjamin Tissoires <benjamin.tissoires@enac.fr>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>