V4L/DVB (11369): v4l2-subdev: add load_fw and use that instead of abusing core->init.

The init callback was used in several places to load firmware. Make a separate
load_fw callback for that. This makes the code a lot more understandable.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>