p54: fix oops caused by bad eeproms

This patch fixes a bug that could occur, if it the eeprom is incomplete or partly corrupted.

e.g:

BUG: unable to handle kernel NULL pointer dereference at 00000008
IP: p54_assign_address+0x108/0x15d [p54common]
Oops: 0002 [#1] SMP
Pid: 12988, comm: phy1 Tainted: P        W  2.6.28-rc6-wl #3
RIP: 0010: p54_assign_address+0x108/0x15d [p54common]
 [...]
Call Trace:
 p54_alloc_skb+0xa3/0xc0 [p54common]
 p54_scan+0x37/0x204 [p54common]
 [...]

---

index 719bdab..234fee3 100644
@@ -1490,7 +1490,7 @@ static int p54_scan(struct ieee80211_hw *dev, u16 mode, u16 dwell,
 
  err:
 	printk(KERN_ERR "%s: frequency change failed\n", wiphy_name(dev->wiphy));
-	kfree_skb(skb);
+	p54_free_skb(dev, skb);
 	return -EINVAL;
 }