HID: bpf: Add support for the default firmware mode of the Huion K20

HID: bpf: Add support for the default firmware mode of the Huion K20

This re-uses the same HID report descriptor that we also use for
the tablet mode after switching the device. The keys send uniquely
identifyable shortcuts so we can map those to buttons.

However the dial and the button inside the dial send events on a
different hidraw node and they are unreliable (e.g. the button does not
get released reliably). So they're ignored in this patch, it's not
worth the effort getting those to work correctly.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Link: https://gitlab.freedesktop.org/libevdev/udev-hid-bpf/-/merge_requests/158
Acked-by: Jiri Kosina <jkosina@suse.com>
Link: https://patch.msgid.link/20250207-bpf-import-2025-02-07-v1-1-6048fdd5a206@kernel.org
Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>