[media] tlg2300: index is unsigned, so never below zero

drivers/media/usb/tlg2300/pd-video.c:891:2: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
drivers/media/usb/tlg2300/pd-video.c:926:2: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]

Cc: Huang Shijie <shijie8@gmail.com>
Cc: Kang Yong <kangyong@telegent.com>
Cc: Zhang Xiaobing <xbzhang@telegent.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>