USB: cdc-acm: downgrade message to debug

commit e4c77070

 upstream.

This failure is so common that logging an error here amounts
to spamming log files.

Reviewed-by: Bruno Thomsen <bruno.thomsen@gmail.com>
Signed-off-by: Oliver Neukum <oneukum@suse.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210311130126.15972-2-oneukum@suse.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>