[media] dvb: split enum from typedefs at frontend.h

Using typedefs is already bad enough, but doing it together
with enum declaration is even worse.

Also, it breaks the scripts at DocBook that would be generating
reference pointers for the enums.

Well, we can't get rid of typedef right now, but let's at least
declare it on a separate line, and let the scripts to generate
the cross-reference, as this is needed for the next DocBook
patches.

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