Some devices, like Perixx Peripad 701 do not work if the feature
"Maximum Contact Number" is not set to the right value.
This patch allows hid-multitouch to control this feature.

If the programmer fills the field maxcontacts in the mt_class,
then the driver will set the feature to this value. It is safe
for current drivers as the feature is read/write in the HID norm
and all devices should implement the norm.
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@enac.fr>
Acked-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>