tegra: clock: Avoid a divide-by-zero error

The clock fix-up for tegra is still present in the code. It causes a
divide-by-zero bug after relocation when chain-loading U-Boot from
coreboot. Fix this by adding a check.

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: 7468676 (ARM: tegra: fix clock_get_periph_rate() for UART clocks)

arch/arm/mach-tegra/clock.c

@@ -339,8 +339,11 @@ unsigned long clock_get_periph_rate(enum periph_id periph_id,
 		 * return value doesn't help. In summary this clock driver is
 		 * quite broken but I'm afraid I have no idea how to fix it
 		 * without completely replacing it.
+		 *
+		 * Be careful to avoid a divide by zero error.
 		 */
-		div -= 2;
+		if (div >= 1)
+			div -= 2;
 		break;
 #endif
 	default: