edid: Fix gcc 7.1 warning

This commit fixes the warning produced by gcc 7.1.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

common/edid.c

@@ -295,7 +295,7 @@ static void edid_print_dtd(struct edid_monitor_descriptor *monitor,
 
 		h_total = h_active + h_blanking;
 		v_total = v_active + v_blanking;
-		if (v_total * h_total)
+		if (v_total > 0 && h_total > 0)
 			vfreq = pixclock / (v_total * h_total);
 		else
 			vfreq = 1; /* Error case */