[media] ir-core: more cleanups of ir-functions.c

cx88 only depends on VIDEO_IR because it needs ir_extract_bits().
Move that function to ir-core.h and make it inline.

Lots of drivers had dependencies on VIDEO_IR when they really
wanted IR_CORE.

The only remaining drivers to depend on VIDEO_IR are bt8xx and
saa7134 (ir_rc5_timer_end is the only function exported by
ir-functions).

Rename VIDEO_IR -> IR_LEGACY to give a hint to anyone writing or
converting drivers to IR_CORE that they do not want a dependency
on IR_LEGACY.

Signed-off-by: David Härdeman <david@hardeman.nu>
Acked-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>