media: subdev: increase V4L2_FRAME_DESC_ENTRY_MAX to 8

V4L2_FRAME_DESC_ENTRY_MAX is currently set to 4. In theory it's possible
to have an arbitrary amount of streams in a single pad, so preferably
there should be no hardcoded maximum number.

However, I believe a reasonable max is 8, which would cover a CSI-2 pad
with 4 streams of pixel data and 4 streams of metadata.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>