[media] gspca: warning fix: index is unsigned, so it will never be below 0

drivers/media/usb/gspca/gspca.c: In function 'vidioc_querybuf':
drivers/media/usb/gspca/gspca.c:1590:6: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]

Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>