serial: imx: Correct comment imx_flush_buffer()

The comment in imx_flush_buffer() states that the state of 4 registers
are to be saved/restored, then only saves and restores 3 registers. The
missing register (UBRC) is read only and thus can't be restored.

Update the comment to reflect reality.
Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/imx.c

@@ -1417,10 +1417,14 @@ static void imx_flush_buffer(struct uart_port *port)
 
 	/*
 	 * According to the Reference Manual description of the UART SRST bit:
+	 *
 	 * "Reset the transmit and receive state machines,
 	 * all FIFOs and register USR1, USR2, UBIR, UBMR, UBRC, URXD, UTXD
-	 * and UTS[6-3]". As we don't need to restore the old values from
-	 * USR1, USR2, URXD, UTXD, only save/restore the other four registers
+	 * and UTS[6-3]".
+	 *
+	 * We don't need to restore the old values from USR1, USR2, URXD and
+	 * UTXD. UBRC is read only, so only save/restore the other three
+	 * registers.
 	 */
 	ubir = readl(sport->port.membase + UBIR);
 	ubmr = readl(sport->port.membase + UBMR);