greybus: operation: add gb_operation_sync_timeout helper

Add gb_operation_sync_timeout convenience function, which allows drivers
to configure the operation timeout.

Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>