arc: fix arc_dma_sync_sg_for_{cpu,device}

These functions should perform the same functionality as calling
arc_dma_sync_single_for_{cpu,device} on each S/G list element.  Ensure
they actually do that by calling arc_dma_sync_single_for_{cpu,device}.
Otherwise we could be passing a different dir argument.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Alexey Brodkin <abrodkin@synopsys.com>
Acked-by: Vineet Gupta <vgupta@synopsys.com>