V4L/DVB (10757): cx18, v4l2-chip-ident: Finish conversion of AV decoder core to v4l2_...

Added a new chip identifer to v4l2-chip-ident for the integrated A/V broadcast
decoder core internal to the CX23418.  Completed separation and encapsulation
of the A/V decoder core interface as a v4l2_subdevice.  The cx18 driver now
compiles and links again.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>