DMA: PL330: Add missing static storage class specifier

Fixes the following sparse warning:
drivers/dma/pl330.c:2542:5: warning: symbol 'add_desc' was not declared. Should it be static?

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>

drivers/dma/pl330.c

@@ -2539,7 +2539,7 @@ static inline void _init_desc(struct dma_pl330_desc *desc)
 }
 
 /* Returns the number of descriptors added to the DMAC pool */
-int add_desc(struct dma_pl330_dmac *pdmac, gfp_t flg, int count)
+static int add_desc(struct dma_pl330_dmac *pdmac, gfp_t flg, int count)
 {
 	struct dma_pl330_desc *desc;
 	unsigned long flags;