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>