drivers: dma_cavs: NULL pointer exception when DMA start called after DMA stop

[sathishkuttan]

In the current implementation of dma_cavs

• Memory for DMA LLI is allocated in DMA configure routine
• DMA start populates the LLIs
• DMA stop frees the LLIs

Hence if the DMA is stopped and restarted, the LLIs are not valid causing an exception.