serial: 8250: Check UART_SCR is writable

Au1x00/RT2800+ doesn't implement the 8250 scratch register (and
this may be true of other h/w currently supported by the 8250 driver);
read back the canary value written to the scratch register to enable
the console h/w restart after resume from system suspend.

Fixes: 4516d50a ("serial: 8250: Use canary to restart console ...")
Reported-by: NMason <slash.tmp@free.fr>
Signed-off-by: NPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_core.c

@@ -3585,6 +3585,7 @@ void serial8250_suspend_port(int line)
 	    port->type != PORT_8250) {
 		unsigned char canary = 0xa5;
 		serial_out(up, UART_SCR, canary);
+		if (serial_in(up, UART_SCR) == canary)
 			up->canary = canary;
 	}