[media] v4l: Correct definition of v4l2_buffer.flags related to cache management

V4L2_BUF_FLAG_NO_CACHE_INVALIDATE and V4L2_BUF_FLAG_NO_CACHE_CLEAN were
define incorrectly in the documentation. Fix this by changing the
documentation to match reality.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>