greybus: loopback: make loopback type input equivalent to protocol type
Sepcifying loopback operation type with a type value that is internal to
the loopback driver is non-intunitive and requires reading code to
understand. Remove confusing duplicate definitions and update code to
accept the greybus-specification function identity defintiions as the
appropriate type values for initiating loopback operations.
See greybus-spec section 10.16.1 'Greybus Loopback Message Types' for a
full list of valid messages to set this type field to.