soc: intel_adsp/ace: fix IDC processing for sched IPI

The Inter-DSP Communication (IDC) is being used to send
sched IPI to other CPU cores. When a core receives an IDC,
it needs to ACK it by clearing the BUSY bit in TDR, and also
needs to the BUSY bit in TDA to signal done after processing.
These two steps are needed to complete one IDC message.
If we do only one (and not both), the other side will not be
able to send another IDC message as the hardware still thinks
the core is processing the IDC message (and thus will not send
another one). So add the step to clear the BUSY bit in TDA so
we can have multiple sched IPIs.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>