bcm283x_pl011: Flush RX queue after setting baud rate

bcm283x_pl011: 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 on Raspberry Pis that use pl011 for serial.

Fixes: d6420f2f43 ("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>