serial: sh-sci: describe locking requirements for invalidating RXDMA

Make sure everyone knows that calling this function needs protection.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20240506114016.30498-8-wsa+renesas@sang-engineering.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sh-sci.c

@@ -1257,6 +1257,7 @@ static int sci_dma_rx_find_active(struct sci_port *s)
 	return -1;
 }
 
+/* Must only be called with uart_port_lock taken */
 static void sci_dma_rx_chan_invalidate(struct sci_port *s)
 {
 	unsigned int i;