staging: greybus: audio: remove redundant slot field

gb_audio_manager_module_descriptor's intf_id field maintains the
information about the interface on which module is connected hence
having an extra slot field is redundant.

Thus remove the slot field and its associated code.

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