[media] zr364xx: urb actual_length is unsigned

drivers/media/usb/zr364xx/zr364xx.c:1010:2: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]

Cc: Antoine Jacquet <royale@zerezo.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>