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.c b/drivers/net/wireless/iwlwifi/iwl-3945.c
index ae5efc3..15de649 100644
--- a/drivers/net/wireless/iwlwifi/iwl-3945.c
+++ b/drivers/net/wireless/iwlwifi/iwl-3945.c
@@ -597,8 +597,8 @@
 	struct iwl3945_rx_frame_stats *rx_stats = IWL_RX_STATS(pkt);
 	struct iwl3945_rx_frame_hdr *rx_hdr = IWL_RX_HDR(pkt);
 	struct iwl3945_rx_frame_end *rx_end = IWL_RX_END(pkt);
-	u16 rx_stats_sig_avg = le16_to_cpu(rx_stats->sig_avg);
-	u16 rx_stats_noise_diff = le16_to_cpu(rx_stats->noise_diff);
+	u16 rx_stats_sig_avg __maybe_unused = le16_to_cpu(rx_stats->sig_avg);
+	u16 rx_stats_noise_diff __maybe_unused = le16_to_cpu(rx_stats->noise_diff);
 	u8 network_packet;
 
 	rx_status.flag = 0;