libertas: convert libertas driver to use an event/cmdresp queue

This patch (co-developed by Dan Williams and Holger Schurig) uses a kfifo
object for events and a swapping buffer scheme for the command response to
preserve the zero-copy semantics of the CF driver and keep memory usage low.
The main thread should only ever touch the buffer indexed by priv->resp_idx,
while the interface code is free to write to the second buffer, then swap
priv->resp_idx under the driver spinlock.  The firmware specs only permit
one in-flight command, so there will only ever be one command response to
process at a time.

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/libertas/rx.c b/drivers/net/wireless/libertas/rx.c
index 09f0230..05af731 100644
--- a/drivers/net/wireless/libertas/rx.c
+++ b/drivers/net/wireless/libertas/rx.c
@@ -145,14 +145,14 @@
 	struct net_device *dev = priv->dev;
 	struct rxpackethdr *p_rx_pkt;
 	struct rxpd *p_rx_pd;
-
 	int hdrchop;
 	struct ethhdr *p_ethhdr;
-
 	const u8 rfc1042_eth_hdr[] = { 0xaa, 0xaa, 0x03, 0x00, 0x00, 0x00 };
 
 	lbs_deb_enter(LBS_DEB_RX);
 
+	BUG_ON(!skb);
+
 	skb->ip_summed = CHECKSUM_NONE;
 
 	if (priv->monitormode)