drivers: serial: nrfx_uarte: Fix race condition in TX path

If interrupt handler contains while loop which depends on
uart_irq_tx_ready() and uart_fifo_fill is called inside this
loop then if TXSTOPPED event occurs while code executes the
loop then uart_irq_tx_ready() will return true but uart_fifo_fill
will fail to write any bytes. That is because fifo_fill_lock is
cleared in handling of TXSTOPPED. To solve that added clearing
the lock inside uart_irq_tx_ready() if STOPPED event is detected
there.

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