selftests/hid: fix failing tablet button tests

An overlook from commit 74452d63 ("selftests/hid: tablets: add
variants of states with buttons"), where I don't use the Enum...

Fixes: 74452d63 ("selftests/hid: tablets: add variants of states with buttons")
Acked-by: Jiri Kosina <jkosina@suse.com>
Link: https://lore.kernel.org/r/20231207-b4-wip-selftests-v1-1-c4e13fe04a70@kernel.org


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