dm: serial: ns16550: Correct logic for checking for character

There is a bug in the logic which checks for an available character. This
can cause invalid characters to be received - this was noticed on
beaglebone. Fix it.
Signed-off-by: NSimon Glass <sjg@chromium.org>
Reviewed-by: NTom Rini <trini@ti.com>

drivers/serial/ns16550.c

@@ -253,7 +253,7 @@ static int ns16550_serial_getc(struct udevice *dev)
 {
 	struct NS16550 *const com_port = dev_get_priv(dev);
 
-	if (!serial_in(&com_port->lsr) & UART_LSR_DR)
+	if (!(serial_in(&com_port->lsr) & UART_LSR_DR))
 		return -EAGAIN;
 
 	return serial_in(&com_port->rbr);