USB: serial: cp210x: add part-number debug printk

Add a part-number debug printk to facilitate debugging.
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/cp210x.c

@@ -2140,6 +2140,8 @@ static void cp210x_determine_type(struct usb_serial *serial)
 		return;
 	}
 
+	dev_dbg(&serial->interface->dev, "partnum = 0x%02x\n", priv->partnum);
+
 	switch (priv->partnum) {
 	case CP210X_PARTNUM_CP2102:
 		cp2102_determine_quirks(serial);