serial: pch: decomission pch_uart_hal_write()

It's horrid and if we inline it into callers, we can get rid of a lot of
sugar around.

So:
* x_char handling becomes a single iowrite8.
* xmit->buf handling is a single loop simply writing characters one by
  one directly from the buf instead of complex cnt_to_end computations.
  Until the buf is empty or fifo size is reached.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220503080808.28332-2-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>