drivers: serial: stm32: rework code for max re-use in wide data support

Restructuring code for poll_in/poll_out/fifo_fill/fifo_read because for
wide data support, all code is identical except the calls to
LL_USART_{ReceiveData8/TransmitData8}.
This allows both implementations, 8 and 9 bit data-width to call a
visitor function, passing the either the 8 bit or 9 bit data-width
function pointer.

Signed-off-by: Jeroen van Dooren <jeroen.van.dooren@nobleo.nl>