HID: 3m: Adjust to sequential MT HID protocol

The multitouch extensions to the HID protocol allows for contact
data to be sent over several reports, which is also the case for
the 3M M2256PW touchscreen. This patch modifies the logic to only
synchronize the input layer when all contacts have been received.
Consequentially, the full 60-finger capacity of the device is enabled.

Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Acked-by: Stephane Chatty <chatty@enac.fr>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>