platform/x86: dell-laptop: Refactor kbd_led_triggers_store()

Return -EINVAL immediately on invalid input, rather then doing
the straight path in an if block and returning -EINVAL at the end
of the function.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>