Revert "serial: mxc: have putc use the TXFIFO"

This reverts commit c7878a04.

Since commit c7878a04 ("serial: mxc: have putc use the TXFIFO"),
serial console corruption can be seen when priting inside board_init().

Revert it to avoid the regression.

Reported-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Tim Harvey <tharvey@gateworks.com>