USB: cdc-acm: downgrade message to debug

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>

drivers/usb/class/cdc-acm.c

@@ -659,7 +659,8 @@ static void acm_port_dtr_rts(struct tty_port *port, int raise)
 
 	res = acm_set_control(acm, val);
 	if (res && (acm->ctrl_caps & USB_CDC_CAP_LINE))
-		dev_err(&acm->control->dev, "failed to set dtr/rts\n");
+		/* This is broken in too many devices to spam the logs */
+		dev_dbg(&acm->control->dev, "failed to set dtr/rts\n");
 }
 
 static int acm_port_activate(struct tty_port *port, struct tty_struct *tty)