[rfc] Propagate manufacturer information from MMKernelDevice to MMBroadbandModem

[Ben Chan]

Hi Aleksander and Dan,

Unlike AT or QMI, MBIM doesn't define a standard command to query
manufacturer information. But the manufacturer information can often
be inferred from the USB vendor ID or retrieved through the
Manufacturer string descriptor at the USB level.

MMBroadbandModemMbim currently uses the plugin name as the
manufacturer, which may not be very useful if the generic plugin is
used. I wonder if it makes sense to let MMBroadbandModemMbim make use
of the manufacturer information, if available, from MMKernelDevice.
Obviously, modem vendors would like us to report a proper manufacturer
name than 'Generic' :)

I've put together a few patches that plumb up the logic, which you
check it out from this github branch:
https://github.com/cbchan/ModemManager/commits/mbim-manufacturer.
I'd like to hear your thoughts on this approach before creating a pull
request. In particular,

* The few MBIM modems I have report the correct manufacturer through
the manufacturer string descriptor, but I suspect some MBIM modems
don't.

*  I find the propagation (i.e. MMKernelDevice -> MMDevice -> MMPlugin
-> MMBaseModem -> MMBroadbandModemMbim) a bit involved and convoluted.
Perhaps there is simpler way to do that?

Thanks,
Ben