dma: mmp_pdma: Staticize mmp_pdma_alloc_descriptor()

mmp_pdma_alloc_descriptor() is used only in this file.
Fix the following sparse warning:

drivers/dma/mmp_pdma.c:359:25: warning: symbol 'mmp_pdma_alloc_descriptor' was not declared. Should it be static?
Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

drivers/dma/mmp_pdma.c

@@ -356,7 +356,8 @@ static dma_cookie_t mmp_pdma_tx_submit(struct dma_async_tx_descriptor *tx)
 	return cookie;
 }
 
-struct mmp_pdma_desc_sw *mmp_pdma_alloc_descriptor(struct mmp_pdma_chan *chan)
+static struct mmp_pdma_desc_sw *
+mmp_pdma_alloc_descriptor(struct mmp_pdma_chan *chan)
 {
 	struct mmp_pdma_desc_sw *desc;
 	dma_addr_t pdesc;