serial: xilinx_uartps: Use helper function hrtimer_update_function()

The field 'function' of struct hrtimer should not be changed directly, as
the write is lockless and a concurrent timer expiry might end up using the
wrong function pointer.

Switch to use hrtimer_update_function() which also performs runtime checks
that it is safe to modify the callback.

Signed-off-by: Nam Cao <namcao@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/af7823518fb060c6c97105a2513cfc61adbdf38f.1738746927.git.namcao@linutronix.de

drivers/tty/serial/xilinx_uartps.c

@@ -454,7 +454,7 @@ static void cdns_uart_handle_tx(void *dev_id)
 
 	if (cdns_uart->port->rs485.flags & SER_RS485_ENABLED &&
 	    (kfifo_is_empty(&tport->xmit_fifo) || uart_tx_stopped(port))) {
-		cdns_uart->tx_timer.function = &cdns_rs485_rx_callback;
+		hrtimer_update_function(&cdns_uart->tx_timer, cdns_rs485_rx_callback);
 		hrtimer_start(&cdns_uart->tx_timer,
 			      ns_to_ktime(cdns_calc_after_tx_delay(cdns_uart)), HRTIMER_MODE_REL);
 	}
@@ -734,7 +734,7 @@ static void cdns_uart_start_tx(struct uart_port *port)
 
 	if (cdns_uart->port->rs485.flags & SER_RS485_ENABLED) {
 		if (!cdns_uart->rs485_tx_started) {
-			cdns_uart->tx_timer.function = &cdns_rs485_tx_callback;
+			hrtimer_update_function(&cdns_uart->tx_timer, cdns_rs485_tx_callback);
 			cdns_rs485_tx_setup(cdns_uart);
 			return hrtimer_start(&cdns_uart->tx_timer,
 					     ms_to_ktime(port->rs485.delay_rts_before_send),