media: staging: atomisp: array underflow in ioctl

I noticed an array underflow in ov5693_enum_frame_size().  The code
looks like this:

	int index = fse->index;

	if (index >= N_RES)
		retur -EINVAL;

fse->index is a u32 that comes from the user.  We want negative values
to be counted as -EINVAL but they aren't.  There are several ways to fix
this but I feel like the best fix for future proofing is to change the
type of N_RES from int to unsigned long to make it the same as if we
were comparing against ARRAY_SIZE().

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>