Add the report_size to struct asus_touchpad_info instead of calculating it.

This is a preparation patch for adding support for the multi-touch touchpad
found on the Medion Akoya E1239T's keyboard-dock, which uses the same
custom multi-touch protocol as the Asus keyboard-docks (same chipset
vendor, Integrated Technology Express / ITE).

The only difference in that the Akoya E1239T keyboard-dock's input-reports
have a 5 byte footer instead of a 1 byte footer, which requires the
report_size to be configurable per touchpad-model.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>