usb: serial: silence all non-critical read errors

If a USB serial device is unplugged while there is an active program
using the device it may spam the logs with -EPROTO (71) messages as it
attempts to retry.

Most serial usb drivers (metro-usb, pl2303, mos7840, ...) only output
these messages for debugging.  The generic driver treats these as
errors.

Change the default output for the generic serial driver from error to
debug to silence these non-critical errors.
Signed-off-by: NJeremiah Mahler <jmmahler@gmail.com>
Signed-off-by: NJohan Hovold <johan@kernel.org>

drivers/usb/serial/generic.c

@@ -373,7 +373,7 @@ void usb_serial_generic_read_bulk_callback(struct urb *urb)
 							__func__, urb->status);
 		return;
 	default:
-		dev_err(&port->dev, "%s - nonzero urb status: %d\n",
+		dev_dbg(&port->dev, "%s - nonzero urb status: %d\n",
 							__func__, urb->status);
 		goto resubmit;
 	}