serial: mxc: Wait for TX completion before reset

serial: mxc: Wait for TX completion before reset

The u-boot console may show some corrupted characters when
printing in board_init() due to reset or baudrate change
of the UART (probe) before the TX FIFO has been completely
drained.

To fix this issue, and in case UART is still running, we now
try to flush the FIFO before proceeding to UART reinitialization.
For this we're waiting for Transmitter Complete bit, indicating
that the FIFO and the shift register are empty.

flushing has a 4ms timeout guard, which is normally more than
enough to consume the FIFO @ low baudrate (9600bps).

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Tested-by: Lothar Waßmann <LW@KARO-electronics.de>
Acked-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Fabio Estevam <festevam@denx.de>