selftests/hid: fix ruff linter complains

rename ambiguous variables l, r, and m, and ignore the return values
of uhdev.get_evdev() and uhdev.get_slot()

Acked-by: Jiri Kosina <jkosina@suse.com>
Link: https://lore.kernel.org/r/20231206-wip-selftests-v2-15-c0350c2f5986@kernel.org


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