HID: multitouch: use the callback "report" instead of sequential events

Nexio 42" devices requires to rely on the HID field Contact
Count to compute the valid values. However, this field is
most of the time at the end of the report, meaning that we
need to get the all report parsed before processing it.

This patch does not introduce functional changes.

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