drivers: hda: insert an empty ";" statement before switch() labels

Only statements can be labeled in C, a declaration is not valid. This is
an FAQ.

While compilers currently in use don't seem to care, the "sparse" static
analyzer complains loudly (and cryptically):

https://github.com/thesofproject/sof/actions/runs/6052920348/job/16427323549



```
drivers/dma/dma_intel_adsp_hda.c:190:17: error: typename in expression
drivers/dma/dma_intel_adsp_hda.c:190:26: error: Expected ; at end of stmt
drivers/dma/dma_intel_adsp_hda.c:190:26: error: got rp
```

Add an empty ";" statement after each label makes `sparse` and probably
others happy.

Also add missing `const` to constants for clarity.

Fixes commit a0263704 ("drivers: hda: use interrupt for timing L1
exit on host DMA")

Signed-off-by: Marc Herbert <marc.herbert@intel.com>