rndis_host: support Hytera digital radios

Hytera makes a range of digital (DMR) radios. These radios can be
programmed to a allow a computer to control them over Ethernet over USB,
either using NCM or RNDIS.

This commit adds support for RNDIS for Hytera radios. I tested with a
Hytera PD785 and a Hytera MD785G. When these radios are programmed to
set up a Radio to PC Network using RNDIS, an USB interface will be added
with class 2 (Communications), subclass 2 (Abstract Modem Control) and
an interface protocol of 255 ("vendor specific" - lsusb even hints "MSFT
RNDIS?").

This patch is similar to the solution of this StackOverflow user, but
that only works for the Hytera MD785:
https://stackoverflow.com/a/53550858



To use the "Radio to PC Network" functionality of Hytera DMR radios, the
radios need to be programmed correctly in CPS (Hytera's Customer
Programming Software). "Forward to PC" should be checked in "Network"
(under "General Setting" in "Conventional") and the "USB Network
Communication Protocol" should be set to RNDIS.

Signed-off-by: Thomas Toye <thomas@toye.io>
Signed-off-by: David S. Miller <davem@davemloft.net>