USB: ftdi_sio: do not claim CDC ACM function

The Microchip vid:pid 04d8:000a is used for their CDC ACM
demo firmware application.  This is a device with a single
function conforming to the CDC ACM specification and with
the intention of demonstrating CDC ACM class firmware and
driver interaction.  The demo is used on a number of
development boards, and may also be used unmodified by
vendors using Microchip hardware.

Some vendors have re-used this vid:pid for other types of
firmware, emulating FTDI chips. Attempting to continue to
support such devices without breaking class based
applications that by matching on interface
class/subclass/proto being ff/ff/00.  I have no information
about the actual device or interface descriptors, but this
will at least make the proper CDC ACM devices work again.
Anyone having details of the offending device's descriptors
should update this entry with the details.

Reported-by: Florian Wöhrl <fw@woehrl.biz>
Reported-by: Xiaofan Chen <xiaofanc@gmail.com>
Cc: stable <stable@vger.kernel.org>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Bruno Thomsen <bruno.thomsen@gmail.com>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>