HID: multitouch: fix protocol for Sitronix 1403:5001

Since the inclusion of this device in hid-multitouch, the device
did not forward any events. Using the serial class makes it working
again.

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