media: dmx.h: split typedefs from structs

Using typedefs inside the Kernel is against CodingStyle, and
there's no good usage here.

Just like we did at frontend.h, at commit 0df289a2
("[media] dvb: Get rid of typedev usage for enums"), let's keep
those typedefs only to provide userspace backward compatibility.

No functional changes.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>