Make it very clear that this header should not be used for i2c drivers that
do not need to be compiled for pre-2.6.26 kernels.

As soon as the minimum supported kernel in the v4l-dvb repository becomes
2.6.26 or up, then this header should be removed entirely.
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>