V4L/DVB (8168): cx18: Upgrade to newer firmware & update cx18 documentation.

Conexant graciously gave us permission to redistribute the
firmware. Update the documentation where the firmware can be
downloaded.

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