Input: atmel_mxt_ts - set byte_offset as signed

The calculations done to obtain byte_offset can result into a negative
number, fix its type.

This patch fixes the following sparse error:

drivers/input/touchscreen/atmel_mxt_ts.c:1481:44: warning: unsigned value that used to be signed checked against zero?
drivers/input/touchscreen/atmel_mxt_ts.c:1479:49: signed value source

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Link: https://patch.msgid.link/20260504-fix-sparse-v1-1-1071137cd280@chromium.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>