qmi: implement detach_shutdown method

The detach_shutdown method is invoked to unconditionally release
an active context.  For QMI, this is equivalent to a call to
deactivate_primary.

This patch makes the callback to deactivate_primary optional and
implements detach_shutdown to simply call it.  When there is no
callback, the stop_net callback notifies ofono about the context
release via an asynchronous ofono_gprs_context_deactivated() call.
1 file changed