HID: elan: Correctly report MT_PRESSURE instead of TOOL_WIDTH

Elan has given me a (GPL-ed) Android driver for their non HID-mt touchpads
to help improve the upstream support.

Acoording to Elan what we are currently reporting as tool-width
really is a per-touch pressure. This always has a maximum of 255, so there
is no need to make the max configurable.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>