sd: switch to using blk_queue_write_cache()

Switch to the newer interface, instead of using blk_queue_flush()
directly.

Signed-off-by: Jens Axboe <axboe@fb.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Martin K. Petersen <martin.petersen@oracle.com>