USB: sierra: avoid CDC class functions on "68A3" devices

Sierra Wireless Direct IP devices using the 68A3 product ID
can be configured for modes including a CDC ECM class function.
The known example uses interface numbers 12 and 13 for the ECM
control and data interfaces respectively, consistent with CDC
MBIM function interface numbering on other Sierra devices.

It seems cleaner to restrict this driver to the ff/ff/ff
vendor specific interfaces rather than increasing the already
long interface number blacklist.  This should be more future
proof if Sierra adds more class functions using interface
numbers not yet in the blacklist.

Cc: <stable@vger.kernel.org>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Johan Hovold <johan@kernel.org>