Fix maxcontacts problem for PWT GeneralTouch multi-touchscreen.

Our device didn't contain HID_DG_CONTACTMAX usage. This usage use to describe
touchscreen's maxcontacts for hid-multitouch.c to get maxcontacts automatic. We
fix the device that driver can get maxcontact from our device, hence it doesn't
need .maxcontact=10. Now there is just one device class can fix all our PWT
touchscreen.
Signed-off-by: Xianhan Yu <aroundight77@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>