rtl8180: improve signal reporting for actual rtl8180 hardware

Adapted from Realtek-provided driver...

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Tested-by: Pauli Nieminen <suokkos@gmail.com>
diff --git a/drivers/net/wireless/rtl818x/rtl818x.h b/drivers/net/wireless/rtl818x/rtl818x.h
index 8522490..22d9384 100644
--- a/drivers/net/wireless/rtl818x/rtl818x.h
+++ b/drivers/net/wireless/rtl818x/rtl818x.h
@@ -193,6 +193,7 @@
 	void (*stop)(struct ieee80211_hw *);
 	void (*set_chan)(struct ieee80211_hw *, struct ieee80211_conf *);
 	void (*conf_erp)(struct ieee80211_hw *, struct ieee80211_bss_conf *);
+	u8 (*calc_rssi)(u8 agc, u8 sq);
 };
 
 /**