mct_u232: Fix use of uninitialized pointer in mct_u323_startup()

My backport of commit 4e9a0b0525 "USB: mct_u232: add sanity checking in
probe" incorrectly added a dev_err() call using port->dev before 'port' was
initialised.  Use the 'serial' parameter to look up the device instead.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/usb/serial/mct_u232.c

@@ -449,7 +449,7 @@ static int mct_u232_startup(struct usb_serial *serial)
 
 	/* check first to simplify error handling */
 	if (!serial->port[1] || !serial->port[1]->interrupt_in_urb) {
-		dev_err(&port->dev, "expected endpoint missing\n");
+		dev_err(&serial->dev->dev, "expected endpoint missing\n");
 		return -ENODEV;
 	}