drivers: uart: nrf_uart: Generate RX_RDY after rx_disable

So far, when rx_disable was called then received data was discarded.
This is currently not according to the API but it is needed if
rx_disabled is called due to out of band information about end of
packet.

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