serial_bcm283x_mu: Flush RX queue after setting baud rate

After the UART was initialized, we may still have bogus data in the
RX queue if it was enabled with incorrect pin muxing before.

So let's flush the RX queue whenever we initialize baud rates.

This fixes a regression with the dynamic pinmuxing code when enable_uart=1
is not set in config.txt.

Fixes: caf2233b ("bcm283x: Add pinctrl driver")
Reported-by: Göran Lundberg <goran@lundberg.email>
Reported-by: Peter Robinson <pbrobinson@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
Tested-by: Peter Robinson <pbrobinson@gmail.com>
Tested-by: Tuomas Tynkkynen <tuomas@tuxera.com>

drivers/serial/serial_bcm283x_mu.c

@@ -51,6 +51,8 @@ struct bcm283x_mu_priv {
 	struct bcm283x_mu_regs *regs;
 };
 
+static int bcm283x_mu_serial_getc(struct udevice *dev);
+
 static int bcm283x_mu_serial_setbrg(struct udevice *dev, int baudrate)
 {
 	struct bcm283x_mu_serial_platdata *plat = dev_get_platdata(dev);
@@ -59,13 +61,17 @@ static int bcm283x_mu_serial_setbrg(struct udevice *dev, int baudrate)
 	u32 divider;
 
 	if (plat->skip_init)
-		return 0;
+		goto out;
 
 	divider = plat->clock / (baudrate * 8);
 
 	writel(BCM283X_MU_LCR_DATA_SIZE_8, &regs->lcr);
 	writel(divider - 1, &regs->baud);
 
+out:
+	/* Flush the RX queue - all data in there is bogus */
+	while (bcm283x_mu_serial_getc(dev) != -EAGAIN) ;
+
 	return 0;
 }