dmaengine: intel_mid_dma: locking and freeing fixes

Two issues are fixed:

1. DMA descriptors are reused so when freeing lli structures
that are linked to them, the pointer must be nulled.

2. midc_scan_descriptors() must be called with the
channel lock held.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>