[review] skip unneeded extra modem references on async actions

[Dan Williams]

On Fri, 2018-06-22 at 17:00 +0200, Aleksander Morgado wrote:
> Hey,
> 
> Since we use GTask and the g_task_get_source_object() is a quick way
> to retrieve the source object owning the async action, there's no
> point in having an additional object reference in the private context
> associated to the task.
> 
> This MR solves this issue in the simple interface:
> https://gitlab.freedesktop.org/mobile-broadband/ModemManager/merge_re
> quests/5

Merged to git master.

Dan