HID: wacom: generic: Support and use 'Custom HID' mode and usages

Wacom's new "MobileStudio Pro" tablets are the first devices in their
branded product line-up to include a usable HID descriptor for the pen
interface. Like prior branded products, the device can operate in one
of two modes: 'Standard HID', and 'Wacom Custom HID'. Although the
first mode is usable by the HID_GENERIC codepath as-is (huzzah!), it is
subject to some restrictions -- most notably pressure being limited
to 2048 levels instead of 8192. To ensure tablets that include support
for Custom HID mode work optimally, we add support for its usages and
switch the device to Custom HID mode if possible.

The usages defined for Custom HID mode are often numerically similar to
their standard HID equivalents, allowing us to write a simple translation
function that takes arbitrary HID usages as input and which returns
the corresponding standard HID usage as output (if one exists). Switching
on this translated usage instead of the actual usage allows the existing
cases to apply to both modes of operation without having to explicitly
define every Custom HID usage.

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>