HID: cmedia: add support for HS-100B mute button

These chips report mute button events in bit 4 of their report without it
being part of the report descriptor.
Use a custom descriptor that maps this bit.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>