serial: uart_nrfx_*: update tx_ready return codes

Update nrfx drivers with the minimum number of bytes that can be sent
in a single call to `uart_fifo_fill`.

Signed-off-by: Jordan Yates <jordan@embeint.com>

drivers/serial/uart_nrfx_uart.c

@@ -878,10 +878,11 @@ static int uart_nrfx_irq_tx_ready_complete(const struct device *dev)
 	 * called after the TX interrupt is requested to be disabled but before
 	 * the disabling is actually performed (in the IRQ handler).
 	 */
-	return nrf_uart_int_enable_check(uart0_addr,
-					 NRF_UART_INT_MASK_TXDRDY) &&
-	       !disable_tx_irq &&
-	       event_txdrdy_check();
+	bool ready = nrf_uart_int_enable_check(uart0_addr,
+					       NRF_UART_INT_MASK_TXDRDY) &&
+		     !disable_tx_irq &&
+		     event_txdrdy_check();
+	return ready ? INT_MAX : 0;
 }
 
 /** Interrupt driven receiver ready function */

drivers/serial/uart_nrfx_uarte.c

@@ -1651,7 +1651,7 @@ static int uarte_nrfx_irq_tx_ready_complete(const struct device *dev)
 		data->int_driven->fifo_fill_lock = 0;
 	}
 
-	return ready;
+	return ready ? data->int_driven->tx_buff_size : 0;
 }
 
 static int uarte_nrfx_irq_rx_ready(const struct device *dev)