drm/i915/audio: add register macros for audio config N value

Have generic macros in line with the rest of the register bit definition
macros instead of a dedicated function in intel_audio.c, and use them.
No functional changes.

Cc: Libin Yang <libin.yang@linux.intel.com>
Reviewed-by: Libin Yang <libin.yang@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/c8709b065ba5cb91b85c54f4e099219e4e68b192.1476111629.git.jani.nikula@intel.com