[media] demux.h: Convert TS filter type into enum

The usage of #define at the kABI is fine, but it doesn't
allow adding a proper description. As those defines deserve
a proper documentation, let's convert them into an enum and
document them at device-drivers DocBook.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>