[media] dvb-core: move dvb_filter out of the DVB core

The dvb_filter.c can hardly be considered as part of the DVB
core. More than half of the code there is commented out by
av7110 and ttusb_dec.

On the latter, just two small helper functions and a struct
definition is used.

Being part of the core means that it would require an
amount of work to fix issues in it, like bad printk's
on it, and to document it on some future, like other kAPI
headers. It simply not worth the effort for something that
seems to be deprecated, as no new drivers use it.

So, move it out of the core, by moving it to pci/ttpci
directory, where av7110 driver is kept, and copy the two
routines used by ttyusb_dec directly into its code.

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