iwlwifi: fix compilation warning when CONFIG_IWLWIFI_DEBUG is not set.

When CONFIG_IWLWIFI_DEBUG is not set and CONFIG_IWLWIFI is set,
we get this compilation warning:
/wireless-next-2.6/drivers/net/wireless/iwlwifi/iwl-agn-rs.c: In
function 'rs_free_sta':
/wireless-next-2.6/drivers/net/wireless/iwlwifi/iwl-agn-rs.c:2425:
warning: unused variable 'priv'

This patch fixes it by adding __maybe_unused attribute.
Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/iwlwifi/iwl-agn-rs.c

@@ -2422,7 +2422,7 @@ static void rs_free_sta(void *priv_r, struct ieee80211_sta *sta,
 			void *priv_sta)
 {
 	struct iwl_lq_sta *lq_sta = priv_sta;
-	struct iwl_priv *priv = priv_r;
+	struct iwl_priv *priv __maybe_unused = priv_r;
 
 	IWL_DEBUG_RATE("enter\n");
 	kfree(lq_sta);