V4L/DVB: ir-core: move subsystem internal calls to ir-core-priv.h

V4L/DVB: ir-core: move subsystem internal calls to ir-core-priv.h

ir-core.h has the kABI to be used by the bridge drivers, when needing to register
IR protocols and pass IR events. However, the same file also contains IR subsystem
internal calls, meant to be used inside ir-core and between ir-core and the raw
decoders.

Better to move those functions to an internal header, for some reasons:

1) Header will be a little more cleaner;

2) It avoids the need of recompile everything (bridge/hardware drivers, etc),
   just because a new decoder were added, or some other internal change were needed;

3) Better organize the ir-core API, splitting the functions that are internal to
   IR core and the ancillary drivers (decoders, lirc_dev) from the features that
   should be exported to IR subsystem clients.

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