drivers: serial: nrf_uart: Disable uart when fifo_fill not used

Driver was failing when interrupt mode was enabled for given
instance but interrupt driven TX part was not used. In that
case uart was not disabled after sending a byte which resulted
in continuous interrupt triggering. Added check for
fifo_fill_lock which is set when uart_fifo_fill is used.

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