iwlagn: fix DMA sync

iwlagn: fix DMA sync

For the RX DMA fix for iwlwifi ("iwlagn: fix RX skb alignment") Luis
pointed out:

> aligned_dma_addr can obviously be > real_dma_addr at this point, what
> guarantees we can use it on our own whim?

I asked around, and he's right, there may be platforms that do not allow
passing such such an address to the DMA API functions. This patch
changes it by using the proper dma_sync_single_range_for_cpu API
invented for this purpose.

Cc: Luis R. Rodriguez <mcgrof@gmail.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>