If buffer is to be accessed by cpu after dma is over, but
between dma mapping and dma unmapping, we should use
dma_sync_single_for_cpu to sync the buffer between cpu with
device. And dma_sync_single_for_device is used to let
device gain the buffer again.

v2: Felix pointed out dma_sync_single_for_device is needed to return
buffer to device if an unsuccessful status bit check is found.
Signed-off-by: NMing Lei <tom.leiming@gmail.com>
Acked-by: NFelix Fietkau <nbd@openwrt.org>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>