blackfin: limit the max memory dma peripheral transfer size to 4 bytes.

Othersize, the bf609 memory dma halts after being enabled.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
diff --git a/arch/blackfin/lib/string.c b/arch/blackfin/lib/string.c
index 44d8c6d..5b7ac0b 100644
--- a/arch/blackfin/lib/string.c
+++ b/arch/blackfin/lib/string.c
@@ -128,10 +128,12 @@
 	unsigned long limit;
 
 #ifdef MSIZE
-	limit = 6;
+	/* The max memory DMA memory transfer size is 32 bytes. */
+	limit = 5;
 	*dshift = MSIZE_P;
 #else
-	limit = 3;
+	/* The max memory DMA memory transfer size is 4 bytes. */
+	limit = 2;
 	*dshift = WDSIZE_P;
 #endif
 
@@ -170,7 +172,8 @@
 	mod = 1 << bpos;
 
 #ifdef PSIZE
-	dsize |= min(3, bpos) << PSIZE_P;
+	/* The max memory DMA peripheral transfer size is 4 bytes. */
+	dsize |= min(2, bpos) << PSIZE_P;
 #endif
 
 	/* Copy sram functions from sdram to sram */