There are already some comments at dvb_math.h that are ready
for DocBook, although not properly formatted.

Convert them, fix some issues and add this file to
the device-drivers DocBook.
Signed-off-by: NMauro Carvalho Chehab <mchehab@osg.samsung.com>
Acked-by: NJonathan Corbet <corbet@lwn.net>