HID: wacom: do not call hid_set_drvdata(hdev, NULL)

This is a common pattern in the HID drivers to reset the drvdata.
However, this is actually already handled by driver core, so there
is no need to do it manually.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Jason Gerecke <jason.gerecke@wacom.com>