HID: multitouch: remove useless last_field_index field

The aim of last_field_index was to detect the end of the report.
With the introduction of .report(), it is not required anymore to
detect it on the fly as we can put it in the right place during the
.report().

The resulting code path is simpler to read because the terminating
condition is not evaluated after each field.

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