greybus: set result in gb_operation_response_send()

Change gb_operation_response_send() so it takes an errno to assign
as an operation's result.  This emphasizes that setting the result
should be the last thing done to an incoming operation before
sending its response.

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