media: use sizeof() instead of V4L2_SUBDEV_NAME_SIZE

Don't rely on a define, let the compiler use the actual
field size.

Remove all uses of the V4L2_SUBDEV_NAME_SIZE define and also
drop the define itself.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Tested-by: Luca Ceresoli <luca.ceresoli@bootlin.com>