drivers: serial: stm32: make poll-out function to pass data by value

Passing data by value is more efficient in this context. As such,
revise `poll_out_fn` and `uart_stm32_poll_out_visitor` to accept
the `out` argument by value instead of by address.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>