HID: lenovo-tpkbd: remove usb dependency

lenovo tpkbd currently relies on the usb interface number to detect
if it is dealing with the touchpad interface or not.
As the report descriptors of the interface 0 does not contain the
button 3, we can use this to remove the need to check for usb.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>