DocBook: Fix kernel-doc to be case-insensitive for private:

On some places, people could use Private: to tag the private fields
of an struct. So, be case-insensitive when parsing "private:"
meta-tag.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>