greybus: operation: make response helper static

Since commit 46ce118a2678 ("operation: refactor response handling")
sending operation responses is handled by greybus core so there is
currently no need to export the response helper.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>