selftests/hid: tablets: define the elements of PenState

This introduces a little bit more readability by not using the raw values
but a dedicated Enum

Acked-by: Jiri Kosina <jkosina@suse.com>
Link: https://lore.kernel.org/r/20231206-wip-selftests-v2-9-c0350c2f5986@kernel.org


Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>