HID: multitouch: do not use pointers towards hid-core

The previous implementation registered a pointer towards hid-core
to the value of contact count. This is not safe and may be difficult
to debug if hid-core ever changes its implementation.
The use of regular indexes is a better choice.

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