HID: wacom: Detect and correct descriptors missing HID_DG_BARRELSWITCH2

ISDv4 devices have long supported reporting data from each of two barrel
switches, but HID_DG_BARRELSWITCH2 itself was only recently standardized.
Prior to its adoption, ISDv4 devices would associate the bit indicating
the state of the second barrel switch with the "Undefined" 0x000D0000
usage. Although most such devices have explicit support, a few use the
HID_GENERIC codepath which ignores the "Undefined" usage.

This patch adds code which detects the presence of a pre-standard second
barrel switch and corrects the usage value so that the HID_GENERIC code
will declare its presence and report its state.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>