Don't exceed more than the 8 bytes of reserved ("needed_headroom") space
when calling pskb_expand_head.  This fixes a bug introduced in linux-3.12.0.

Signed-off-by: Mark Lord <mlord@pobox.com>

--- ax88179_178a.c.orig	2013-11-03 18:41:51.000000000 -0500
+++ linux/drivers/net/usb/ax88179_178a.c	2013-11-17 13:47:26.127971404 -0500
@@ -1183,10 +1183,10 @@
 	if (((skb->len + 8) % frame_size) == 0)
 		tx_hdr2 |= 0x80008000;	/* Enable padding */
 
-	headroom = skb_headroom(skb) - 8;
+	headroom = skb_headroom(skb);
 
-	if ((skb_header_cloned(skb) || headroom < 0) &&
-	    pskb_expand_head(skb, headroom < 0 ? 8 : 0, 0, GFP_ATOMIC)) {
+	if ((skb_header_cloned(skb) || headroom < 8) &&
+	    pskb_expand_head(skb, headroom < 8 ? 8 - headroom : 0, 0, GFP_ATOMIC)) {
 		dev_kfree_skb_any(skb);
 		return NULL;
 	}