Bluetooth: Audio: VOCS use bt_vocs instead of bt_vocs_server

Modify vocs.c to use the bt_vocs struct instead of the
bt_vocs_server struct. This is done so that there is less
difference between the internal struct usage and the struct
type used in the API.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>