swiotlb: fix comment on swiotlb_bounce()

Fix the comment as swiotlb_bounce() is used to copy from original dma
location to swiotlb buffer during swiotlb_tbl_map_single(), while to
copy from swiotlb buffer to original dma location during
swiotlb_tbl_unmap_single().

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

kernel/dma/swiotlb.c

@@ -385,7 +385,7 @@ void __init swiotlb_exit(void)
 }
 
 /*
- * Bounce: copy the swiotlb buffer back to the original dma location
+ * Bounce: copy the swiotlb buffer from or back to the original dma location
  */
 static void swiotlb_bounce(phys_addr_t orig_addr, phys_addr_t tlb_addr,
 			   size_t size, enum dma_data_direction dir)