HID: input: tag touchscreens as such if the physical is not there

Some devices (Elan, Synaptics...) are sometimes not setting a physical
in their finger collections. hid-input will consider them to be pen
devices, leading to some wrong behavior in user space.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Reviewed-by: Ping Cheng <ping.cheng@wacom.com>
Acked-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>