[POWERPC] Fix default clock for udbg_16550

This patch makes it possible to provide 0 as the clock value for
udbg_16550, making it default to the standard 1.8432Mhz clock

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/udbg_16550.c

@@ -81,10 +81,14 @@ static int udbg_550_getc(void)
 void udbg_init_uart(void __iomem *comport, unsigned int speed,
 		    unsigned int clock)
 {
-	unsigned int dll, base_bauds = clock / 16;
+	unsigned int dll, base_bauds;
 
+	if (clock == 0)
+		clock = 1843200;
 	if (speed == 0)
 		speed = 9600;
+
+	base_bauds = clock / 16;
 	dll = base_bauds / speed;
 
 	if (comport) {