msi-wmi: Add mute key support

Add new MUTE key seen on Medion Akoya AIO PC P4010D using MSI motherboard
(Product Name: MS-7621)

Reported-and-tested-by: Mark Huijgen <mark.sf.net@huijgen.tk>
Signed-off-by: Anisse Astier <anisse@astier.eu>
Signed-off-by: Matthew Garrett <mjg@redhat.com>