greybus: use "operation_id" for certain values

A message header contains a field "id" that is an operation id.
Since the field doesn't identify the message itself, rename this
field so it's clearer what it's referring to.

Similarly gb_pending_operation_find() has a parameter "id" that
is really an operation id, so rename that as well.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>