drivers/uart: stm32: fix dead lock on poll_out

A dead lock could happen if 2 threads with differents priorities use
poll_out. In fact, the lock data->tx_lock could be lock by a thread with
lower priority and then a thread with higher priority can't take the
lock. There was a race condition here:

/* Wait for TXE flag to be raised */
while (1) {
	if (atomic_cas(&data->tx_lock, 0, 1)) {
		/* !!!!!!!! RACE CONDITION !!!!!!!!!!!!!!
		if (LL_USART_IsActiveFlag_TXE(UartInstance)) {
			break;
		}
		atomic_set(&data->tx_lock, 0);
	}
}

To fix race condition, the interrupts are locked in poll_out.

Signed-off-by: Julien D'ascenzio <julien.dascenzio@paratronic.fr>