Input: wacom - make sure touch_max is set for touch devices

Old single touch Tablet PCs do not have touch_max set at
wacom_features. Since touch device at lease supports one
finger, assign touch_max to 1 when touch usage is defined
in its HID Descriptor and touch_max is not pre-defined.

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