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>