intel_mid_dma: fix the WARN_ONs

Moved the WARN_ON to BUG_ON, as WARN_ON if hit,
can cause null pointer derefrences

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>