dma: mmp_pdma: Fix physical channel memory allocation size

Use sizeof(*var) instead of sizeof(type) when calling devm_k*alloc().
This avoids using the wrong type as was done to allocate the physical
channels array.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>