[media] v4l: xilinx: harmless buffer overflow

My static checker warns that the name of the port can be 15 characters
when you consider the NUL terminator and that's one more than the 14
characters in name[].  Maybe it's an off-by-one?

It's unlikely that we hit the limit and even if we do the overflow will
only affect one of the two bytes of padding so it's harmless.  Still
let's fix it and also change the sprintf() to snprintf().

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>