media: subdev: Drop implicit zeroing of stream field

Now that the kernel drivers have been fixed to initialize the stream
field, and we have the client capability which the userspace uses to say
it has initialized the stream field, we can drop the implicit zeroing of
the stream field in the various check functions.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>