[media] use a function for DVB media controller register

This is really a simple function, but using it avoids to have
if's inside the drivers.

Also, the kABI becomes a little more clearer.

This shouldn't generate any overhead, and the type check
will happen when compiling with MC DVB enabled.

So, let's do it.

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