p54pci: fix serious sparse warning

This patch fixes a bug which was just recently introduced by ("p54pci: prevent stuck rx-ring on slow system").

make M=drivers/net/wireless/p54 C=2 CF=-D__CHECK_ENDIAN__
CHECK drivers/net/wireless/p54/p54pci.c
drivers/net/wireless/p54/p54pci.c:143:11: warning: cast to restricted __le32
CC [M] drivers/net/wireless/p54/p54pci.o

---

index 86f3e9a..679da7e 100644
@@ -140,7 +140,7 @@ static void p54p_refill_rx_ring(struct ieee80211_hw *dev,
 
 	idx = le32_to_cpu(ring_control->host_idx[ring_index]);
 	limit = idx;
-	limit -= le32_to_cpu(index);
+	limit -= index;
 	limit = ring_limit - limit;
 
 	i = idx % ring_limit;