greybus: audio: Split helper APIs based on stream direction

Now, module is enabled/disabled based on widget event only and not
during startup/shutdown callbacks. Thus, we needn't compare codec_state
in enable/disable module helper APIs. Also, these can be further
simplified based on stream direction.

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