drivers: serial: nrfx_uarte: Improve RX FIFO flushing workaround

Improved a workaround. When FIFO is not empty then FLUSHRX
task will generate RXSTARTED event to indicated that DMA transfer
was started. This property can be used to detect when FIFO was
not empty and workaroud a HW bug where RX.AMOUNT register is not
updated after flushing empty FIFO.

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