The HID descriptor for HUION tablets was limited to 13 buttons. However,
there are devices with more buttons in their frames.

So far, the device with more buttons released by HUION is the Huion
Kamvas Pro 24 (GT-240, QHD) [1], with 20 buttons.

Tweak the HID descriptor to support it.

Link: https://gitlab.freedesktop.org/libinput/libinput/-/issues/989  [1]
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>