driver: uart: native: fix stop_bits/databits mix

Correct a trivial bug and doxygen documentation error
in which data_bits was missused instead of stop_bits.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>
Signed-off-by: Mateusz Grzywacz <amateusz.grzywacz@gmail.com>