drivers: nrf: uarte: rx flush amount calculation

The NRF UARTE has an undocumented feature that when you flush the RX
FIFO, the RXAMOUNT register is not cleared to zero if the FIFO is in
fact empty. This fix is correcting something that was most likely a
typo.

Signed-off-by: Petri Oksanen <petri@iote.ai>