iwlwifi: fix compile warnings when compiling without debug

Fixes:
CC [M]  drivers/net/wireless/iwlwifi/iwl-agn-rs.o
drivers/net/wireless/iwlwifi/iwl-agn-rs.c: In function ‘rs_get_rate’:
drivers/net/wireless/iwlwifi/iwl-agn-rs.c:2419: warning: unused variable ‘priv’
CC [M]  drivers/net/wireless/iwlwifi/iwl-sta.o
drivers/net/wireless/iwlwifi/iwl-sta.c: In function ‘iwl_send_add_sta’:
drivers/net/wireless/iwlwifi/iwl-sta.c:197: warning: unused variable ‘sta_id’

drivers/net/wireless/iwlwifi/iwl-3945.c: In function ‘iwl3945_rx_reply_rx’:
drivers/net/wireless/iwlwifi/iwl-3945.c:601: warning: unused variable ‘rx_stats_noise_diff’
drivers/net/wireless/iwlwifi/iwl-3945.c:600: warning: unused variable ‘rx_stats_sig_avg’
drivers/net/wireless/iwlwifi/iwl-3945-rs.c: In function ‘rs_get_rate’:
drivers/net/wireless/iwlwifi/iwl-3945-rs.c:650: warning: unused variable ‘priv’

Reported-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
diff --git a/drivers/net/wireless/iwlwifi/iwl-3945-rs.c b/drivers/net/wireless/iwlwifi/iwl-3945-rs.c
index 9e411dc..80e9bbc 100644
--- a/drivers/net/wireless/iwlwifi/iwl-3945-rs.c
+++ b/drivers/net/wireless/iwlwifi/iwl-3945-rs.c
@@ -647,7 +647,7 @@
 	unsigned long flags;
 	u16 rate_mask = sta ? sta->supp_rates[sband->band] : 0;
 	s8 max_rate_idx = -1;
-	struct iwl_priv *priv = (struct iwl_priv *)priv_r;
+	struct iwl_priv *priv __maybe_unused = (struct iwl_priv *)priv_r;
 	struct ieee80211_tx_info *info = IEEE80211_SKB_CB(skb);
 
 	IWL_DEBUG_RATE(priv, "enter\n");