HID: multitouch: fix handling of buggy reports descriptors for Dell ST2220T

LG multitouch panels, such as the one found in Dell ST2220T, has buggy
reports descriptors. With the previous implementation, it was impossible
to rely on the reports descriptors to determine how the different
touches are emitted from the device.

This patch changes the splitting of the different touches in the report
in a more robust way.

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