V4L/DVB (10250): cx25840: fix regression: fw not loaded on first use

With the conversion to v4l2_subdev one bit of code was accidentally dropped:
on receiving the first command the driver has to load the fw. A new init()
command was introduced to do that explicitly for bridge drivers that are
converted to use v4l2_subdev, but old drivers that are not yet converted
no longer worked.

This patch fixes this regression for these old drivers.

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