staging: greybus: audio: Rename cport with intf_id

gb_audio_manager_module_descriptor's cport field is actually used to
manage and pass interface id to user space.

Thus rename gb_audio_manager_module_descriptor's 'cport' field and
few other things to avoid confusion.

Signed-off-by: Pankaj Bharadiya <pankaj.bharadiya@gmail.com>
Reviewed-by: Vaibhav Agarwal <vaibhav.sr@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>