platform/x86: dell-laptop: Protect kbd_state against races

The kbd led code has multiple entry points each of which modifies the
kbd_state by reading it, modifying a copy, writing the copy and on
error setting the modified copy writing back the original state.

This is racy, so add a mutex protection the read-modify-write cycle
on each of the entry points.

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>