platform/x86: panasonic-laptop: Add write support to mute

Add write support to the mute platform device

Signed-off-by: Kenneth Chan <kenneth.t.chan@gmail.com>
Link: https://lore.kernel.org/r/20200821181433.17653-7-kenneth.t.chan@gmail.com


Signed-off-by: Hans de Goede <hdegoede@redhat.com>