greybus: audio_manager: use an 'ida' for the module id

Every time we hotplug an audio module, we get a new audio module id.  We
should recycle them instead of just constantly incrementing the number
so we don't see things like:
	[178016.832580] Created audio module #6124
in the kernel logs.

Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>