staging: greybus: operation: add private data with get/set accessors

Asynchronous operation completion handler's lives are made easier if there
is a generic pointer that can store private data associated with the
operation. This patch adds a pointer field to struct gb_operation and
get/set methods to access that pointer.

Signed-off-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Cc: Johan Hovold <johan@kernel.org>
Cc: Alex Elder <elder@kernel.org>
Cc: Mitch Tasman <tasman@leaflabs.com>
Cc: greybus-dev@lists.linaro.org
Cc: devel@driverdev.osuosl.org
Cc: linux-kernel@vger.kernel.org
Reviewed-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>