media: cx25840: add kernel-doc description of struct cx25840_state

This commit describes a device instance private data of the driver
(struct cx25840_state) in a kernel-doc style comment.

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>