drivers: serial: nrfx_uarte: Fix RX path without low power modes

RX FIFO flushing on RXTO event should only be performed when
UARTE peripheral might be disable during inactivity and that
happens when low power modes is enabled or when device runtime
PM is used. Flushing was incrementing flush_cnt which was not
used (flushed data is not copied to the next buffer) which
was causing data loss and invalid RX data length reporting.

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