Input: zforce_ts - make parsing of contacts less confusing

Input: zforce_ts - make parsing of contacts less confusing

Zforce touch data packet consists of a byte representing number of
contacts followed by several chunks with length of 9 bytes representing
each contact. Instead of accounting for the leading byte by increasing
offset of each field in contacts by one introduce a pointer to contact
data and point it appropriately. This avoids awkward constructs like:

	point.prblty = payload[9 * i + 9];

which makes it seem like there is off-by-one error, in favor of more
straightforward:

	point.prblty = p[8];

Tested-by: Andreas Kemnade <andreas@kemnade.info> # Tolino Shine2HD
Link: https://lore.kernel.org/r/20240824055047.1706392-11-dmitry.torokhov@gmail.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>