greybus: audio: Build audio module conditionally

Added CONFIG_XXX flag check before compiling audio module.
Once we add dynamic DAI link registration from audio driver,
this check wil be required to avoid compilation failures with
other kernel revisions.

Also, renamed header file to better align with .c file name.

Signed-off-by: Vaibhav Agarwal <vaibhav.agarwal@linaro.org>
Signed-off-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>