blackfin: clean up string bfin_dma_5xx after rename.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Bob Liu <lliubbo@gmail.com>

arch/blackfin/lib/ins.S

@@ -66,7 +66,7 @@
  *  - turns interrupts off every loop (low overhead, but longer latency)
  *  - DMA version, which do not suffer from this issue. DMA versions have
  *      different name (prefixed by dma_ ), and are located in
- *      ../kernel/bfin_dma_5xx.c
+ *      ../kernel/bfin_dma.c
  * Using the dma related functions are recommended for transferring large
  * buffers in/out of FIFOs.
  */