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.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>