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

Reported-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/p54/p54pci.c

@@ -139,7 +139,7 @@ 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;