Input: st1232 - simplify parsing of read buffer

Avoid complex 2-variable loop when parsing touchscreen data to make the
code clearer.

Acked-by: Martin Kepplinger <martink@posteo.de>
Tested-by: Matthias Fend <Matthias.Fend@wolfvision.net>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>