usb: cdc-wdm: ignore speed change notifications

The only notification supported by the Device Management class is
Response Available. But this driver is also used as a subdriver of
other CDC classes, allowing notifications like Speed Change and
Network Connection. This results in log messages which are only
confusing to an end user:

 [66255.801874] cdc_mbim 1-3:1.5: unknown notification 42 received: index 5 len 8

These drivers use cdc-wdm as a subdriver to allow access to an
embedded management protocol, and all management is expected to
use this protocol. There is therefore no need to handle any of
these optional CDC notifications. Instead we can let the cdc-wdm
driver recognize them and log a debug level message instead of an
error.

Reported-by: Rob Gardner <robmatic@gmail.com>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>