Fix pl08x warnings

drivers/dma/amba-pl08x.c: In function 'pl08x_desc_free':
drivers/dma/amba-pl08x.c:1173:2: warning: passing argument 1 of 'dma_descriptor_unmap' from incompatible pointer type
include/linux/dmaengine.h:476:91: note: expected 'struct dma_async_tx_descriptor *' but argument is of type 'struct pl08x_txd *'

Fixes: d38a8c62 ("dmaengine: prepare for generic 'unmap' data")
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>