MN10300: Introduce barriers to replace removed volatiles in gdbstub 16550 driver

Introduce into the MN10300 gdbstub 16550 driver a couple of barrier() calls to
replace the removed volatility of the input/output index variables for the Rx
ring buffer.  A previous patch added them into the on-chip serial port driver.
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/mn10300/kernel/gdb-io-serial.c

@@ -99,6 +99,7 @@ int gdbstub_io_rx_char(unsigned char *_ch, int nonblock)
  try_again:
 	/* pull chars out of the buffer */
 	ix = gdbstub_rx_outp;
+	barrier();
 	if (ix == gdbstub_rx_inp) {
 		if (nonblock)
 			return -EAGAIN;
@@ -110,6 +111,7 @@ int gdbstub_io_rx_char(unsigned char *_ch, int nonblock)
 
 	ch = gdbstub_rx_buffer[ix++];
 	st = gdbstub_rx_buffer[ix++];
+	barrier();
 	gdbstub_rx_outp = ix & 0x00000fff;
 
 	if (st & UART_LSR_BI) {