mlx4_core: Fix dma_sync_single_for_cpu() with matching for_device() calls

Commit 5d23a1d2 ("net: replace dma_sync_single with
dma_sync_single_for_cpu") replaced uses of the deprectated function
dma_sync_single() with calls to dma_sync_single_for_cpu().  However,
to be correct, the code should do a sync for_cpu() before touching the
memory and for_device() after it's done.

Signed-off-by: Roland Dreier <rolandd@cisco.com>