ath9k: avoid the copy skb->cb on every RX'd skb

The skb->cb (control buffer, 48 bytes) is available to the skb
upon skb allocation. You can fill it up imediately after skb
allocation. ath9k was copying onto the skb->cb the data from the
processed skb for mac80211 from a stack struct ieee80211_rx_status
structure. This is unnecessary, instead use the skb->cb for the
rx status immediately after the skb becomes available and DMA
synched.

Additionally, avoid the copy of the skb->cb also for virtual wiphys
as skb_copy() will copy over the skb->cb for us as well.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>