V4L/DVB (12108): v4l2-i2c-drv.h: add comment describing when not to use this header.

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>