serial: sh-sci: Clarify enable_ms/break_ctl comments.

Technically there's nothing we can do for either of these, so update the
comments to reflect this, rather than infering that there's additional
work to be done.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>
diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
index 3bea91b..cd13792 100644
--- a/drivers/tty/serial/sh-sci.c
+++ b/drivers/tty/serial/sh-sci.c
@@ -1465,12 +1465,17 @@
 
 static void sci_enable_ms(struct uart_port *port)
 {
-	/* Nothing here yet .. */
+	/*
+	 * Not supported by hardware, always a nop.
+	 */
 }
 
 static void sci_break_ctl(struct uart_port *port, int break_state)
 {
-	/* Nothing here yet .. */
+	/*
+	 * Not supported by hardware. Most parts couple break and rx
+	 * interrupts together, with break detection always enabled.
+	 */
 }
 
 #ifdef CONFIG_SERIAL_SH_SCI_DMA