[Task Processor] API cleanup

Signed-off-by: Regis Merlino <regis.merlino@intel.com>
[Tasks] Structure re-arch for msu_task_t and msu_async_cb_data_t

Signed-off-by: Regis Merlino <regis.merlino@intel.com>

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[Tasks] Simplify many function prototypes following the struct re-arch for tasks

Signed-off-by: Regis Merlino <regis.merlino@intel.com>