media: em28xx: constify a new function

em28xx_duplicate_dev() is static. This were supposed to be
merged on the last patch, but somehow, I forgot "-a" when
I called git commit --amend.

Fixes: be7fd3c3a8 ("media: em28xx: Hauppauge DualHD second tuner functionality")
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/usb/em28xx/em28xx-cards.c

@@ -3435,7 +3435,7 @@ static int em28xx_init_dev(struct em28xx *dev, struct usb_device *udev,
 	return 0;
 }
 
-int em28xx_duplicate_dev(struct em28xx *dev)
+static int em28xx_duplicate_dev(struct em28xx *dev)
 {
 	int nr;
 	struct em28xx *sec_dev = kzalloc(sizeof(*sec_dev), GFP_KERNEL);