Input: wacom - retrieve maximum number of touch points

From the HID usage table when it is supported.

Tested-by: Jason Gerecke <killertofu@gmail.com>
Signed-off-by: Chris Bagwell <chris@cnpbagwell.com>
Signed-off-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>