greybus: encapsulate operation result access

Hide the setting and getting of the operation result (stored in
operation->errno) behind a pair of accessor functions.  Only the
operation core should be setting the result, but operations that
complete asynchronously will need access to the result so expose
the function that provides that.

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