af_hdcd: hdcd_scan() and hdcd_integrate() handle stereo and single channel

New versions of hdcd_scan() and hdcd_integrate() that also do the
work of hdcd_scan_stereo() and hdcd_integrate_stereo().

Some code split into previously separate functions to remove
duplication is now merged back into each function in the single
place where it is used.
Signed-off-by: Burt P <pburt0@gmail.com>