drivers: serial: ns16550: apply `__maybe_unused` to `ns16550_out,inword`

Instead of disabling code with a specific compilation switch, enable it
and apply `__maybe_unused` in the `ns16550_outword` and `ns16550_inword`
functions to make the codebase cleaner.

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