powerpc: Unsigned speed cannot be negative in udbg_16559.c

"unsigned int" speed cannot be negative, it's thus pointless
to test if it is.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/udbg_16550.c

@@ -142,7 +142,7 @@ unsigned int udbg_probe_uart_speed(void __iomem *comport, unsigned int clock)
 	speed = (clock / prescaler) / (divisor * 16);
 
 	/* sanity check */
-	if (speed < 0 || speed > (clock / 16))
+	if (speed > (clock / 16))
 		speed = 9600;
 
 	return speed;