USB: cdc-acm: Add pseudo modem without AT command capabilities

cdc-acm.c : Manage pseudo-modem without AT commands capabilities
  Enable to drive electronic simple gadgets based on microcontrolers.
  The Interface descriptor is like this:
    bInterfaceClass         2 Communications
    bInterfaceSubClass      2 Abstract (modem)
    bInterfaceProtocol      0 None

Signed-off-by: Philippe Corbes <philippe.corbes@gmail.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>