input/touchscreen: imagis: Correct the maximum touch area value

As specified in downstream IST3038B driver and proved by testing,
the correct maximum reported value of touch area is 16.

Signed-off-by: Markuss Broks <markuss.broks@gmail.com>
Signed-off-by: Karel Balej <balejk@matfyz.cz>
Link: https://lore.kernel.org/r/20240301164659.13240-2-karelb@gimli.ms.mff.cuni.cz
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>