ath: add common ath_rxbuf_alloc() and make ath9k use it

Turns out ath5k and ath9k can share the same helper to
allocates RX skbs. We allocate skbs aligned to the cache line
size. This requirement seems to have come from AR5210; when
this was not done it seems sometimes we'd get bogus data. I'm
also told it may have been a performance enhancement
consideration. In the end I can't be sure we can remove this
on new hardware so just keep this and start sharing it through
ath.ko.

Make ath9k start using this, ath5k is next.

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

drivers/net/wireless/ath/ath9k/ath9k.h

@@ -25,6 +25,7 @@
 #include "hw.h"
 #include "rc.h"
 #include "debug.h"
+#include "../ath.h"
 
 struct ath_node;
 
@@ -532,6 +533,8 @@ struct ath_softc {
 	struct ieee80211_hw *hw;
 	struct device *dev;
 
+	struct ath_common common;
+
 	spinlock_t wiphy_lock; /* spinlock to protect ath_wiphy data */
 	struct ath_wiphy *pri_wiphy;
 	struct ath_wiphy **sec_wiphy; /* secondary wiphys (virtual radios); may
@@ -564,7 +567,6 @@ struct ath_softc {
 	u32 sc_flags; /* SC_OP_* */
 	u16 curtxpow;
 	u16 curaid;
-	u16 cachelsz;
 	u8 nbcnvifs;
 	u16 nvifs;
 	u8 tx_chainmask;