video: fbdev: avoid int-in-bool-context warning

gcc-7 suspects this code might be wrong because we use the
result of a multiplication as a bool:

drivers/video/fbdev/core/fbmon.c: In function 'fb_edid_add_monspecs':
drivers/video/fbdev/core/fbmon.c:1051:84: error: '*' in boolean context, suggest '&&' instead [-Werror=int-in-bool-context]

It's actually fine, so let's add a comparison to zero to make
that clear to the compiler too.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>

drivers/video/fbdev/core/fbmon.c

@@ -1048,7 +1048,7 @@ void fb_edid_add_monspecs(unsigned char *edid, struct fb_monspecs *specs)
 
 	for (i = 0; i < (128 - edid[2]) / DETAILED_TIMING_DESCRIPTION_SIZE;
 	     i++, block += DETAILED_TIMING_DESCRIPTION_SIZE)
-		if (PIXEL_CLOCK)
+		if (PIXEL_CLOCK != 0)
 			edt[num++] = block - edid;
 
 	/* Yikes, EDID data is totally useless */