drivers: uart: Cleanup not supported API handling

Fixed discrepancy between documentation and actual returned
error code when API is not enabled or not supported by a
device. Added detection of case when device does not implement
uart_callback_set but ASYNC api is enabled. Returning -ENOSYS in
that case.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>