greybus: complete overflow responses

If a response arrives for an operation request and the allotted
buffer isn't big enough we report the error, but we don't finish
processing the response.

Instead, set the operation result, but then finish processing
the response (no different from any other operation error).

This will allow the normal completion handling to occur for
this error case.

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