[review] skip unneeded extra modem references on async actions

[Aleksander Morgado]

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_requests/5

Comments?

-- 
Aleksander
https://aleksander.es