Try to detect broken AT-control based modems

[Aleksander Morgado]

A short series of patches to improve the detection of broken modems. In this case, we try to detect modems that are still exposed in the USB layer but which don't reply to any of our AT commands.

The first patch is a small cleanup that I wouldn't mind to get included right away in git master. The second patch maybe we should delay it until 1.8 is released?

[PATCH 1/2] base-modem: improve broken modem detection logic
[PATCH 2/2] base-modem: setup a maximum number of timeouts also for USB modems