selftests/hid: use a enum class for the different button types

selftests/hid: use a enum class for the different button types

Instead of multiple spellings of a string-provided argument, let's make
this a tad more type-safe and use an enum here.

And while we do this fix the two wrong devices:
- elan_04f3_313a (HP ZBook Fury 15) is discrete button pad
- dell_044e_1220 (Dell Precision 7740) is a discrete button pad

Equivalent hid-tools commit
https://gitlab.freedesktop.org/libevdev/hid-tools/-/commit/8300a55bf4213c6a252cab8cb5b34c9ddb191625

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>