media: isppreview: fix __user annotations

The 'from' variable at preview_config() expects an __user * type.

However, the logic there does:

    from = *(void * __user *) ((void *)cfg + attr->config_offset);

With actually means a void pointer, pointing to a void __ user
pointer. When the first pointer is de-referenced with *(foo),
the type it returns is "void *" instead of "void __user *".

Change it to:
    from = *(void __user **) ((void *)cfg + attr->config_offset);

in order to obtain, when de-referenced, a void __user pointer,
as desired.

That prevent those warnings:
   drivers/media/platform/omap3isp/isppreview.c:893:45: warning: incorrect type in initializer (different address spaces)
   drivers/media/platform/omap3isp/isppreview.c:893:45:    expected void [noderef] <asn:1>*from
   drivers/media/platform/omap3isp/isppreview.c:893:45:    got void *[noderef] <asn:1><noident>
   drivers/media/platform/omap3isp/isppreview.c:893:47: warning: dereference of noderef expression

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>