Use dma_map_sg() instead of pci_map_sg() in comments.

Link: https://lore.kernel.org/r/20210925125324.1760-3-caihuoqing@baidu.comSigned-off-by: Cai Huoqing <caihuoqing@baidu.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>