ath9k-htc:respect usb buffer cacheline alignment in reg out path

In ath9k-htc register out path, ath9k-htc will pass skb->data into
usb hcd and usb hcd will do dma mapping and unmapping to the buffer
pointed by skb->data, so we should pass a cache-line aligned address.

This patch replace __dev_alloc_skb with alloc_skb to make skb->data
pointed to a cacheline aligned address simply since ath9k-htc does not
skb_push on the skb and pass it to mac80211, also use kfree_skb to free
the skb allocated by alloc_skb(we can use kfree_skb safely in hardirq
context since skb->destructor is NULL always in the path).

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>