cdc-acm: log message for serial state notification

Adds a similar log message to USB_CDC_NOTIFY_SERIAL_STATE as it is
already done with USB_CDC_NOTIFY_NETWORK_CONNECTION.

Signed-off-by: Tobias Herzog <t-herzog@gmx.de>
Acked-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>