HID: multitouch: fix LG Melfas touchscreen

The LG Melfas touchscreen has a bad firmware where it declares the Contact ID
field as constant while it shouldn't. This messes up the autodetection and the
reporting of the events by hid-multitouch given that hid-input ignores constant
fields.

The autodetection is simply worked around by manually adding the device to
hid_have_special_driver[].

The processing of the events requires either a report fixup, or some specific
case handling. Given that the report fixup would require to basically rewrite
all the report descriptor, I went for the programatic way of fixing that after
the report descriptors are loaded.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=1416181
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>