serial: lpuart: Call local version of setbrg and putc directly

There is no need to go through serial driver subsystem, instead
call the driver's setbrg and putc routines directly.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>