USB: cdc-acm: work around some broken devices

This patch introduces a work around for cdc-acm devices which are
low speed contrary to the specification, which requires bulk endpoints
which are banned in low speed and converted by usbcore to virtual
interrupt endpoints if they are used nevertheless.

Signed-off-by: Arseniy Lartsev <ars3n@yandex.ru>
Cc: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>