USB: DL100B webmail notifier: initialize return value

If case of an unknown usb_device_id->driver_info (which could only
occur if the info got corrupted somewhere outside the usbled driver),
a debug message depended on an uninitialized value. This was harmless,
but ugly, and gets fixed with this patch.

Signed-off-by: Melchior FRANZ <mfranz@aon.at>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>