HID: hid-multitouch: Auto detection of maxcontacts

This patch enables support of autodetection of maxcontacts.
When adding support for a new device, one is now able to let
the device tell how many contacts it supports, or to manually
set the value if the device happens to provide wrong information.

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