wifi: rtlwifi: add calculate_bit_shift()

There are many same functions like _rtl88e_phy_calculate_bit_shift(),
_rtl92c_phy_calculate_bit_shift() and so on. And these functions can
cause undefined bitwise shift behavior. Add calculate_bit_shift() to
replace them and fix undefined behavior in subsequent patches.
Signed-off-by: Su Hui <suhui@nfschina.com>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20231219065739.1895666-2-suhui@nfschina.com

drivers/net/wireless/realtek/rtlwifi/wifi.h

@@ -3052,4 +3052,11 @@ static inline struct ieee80211_sta *rtl_find_sta(struct ieee80211_hw *hw,
 	return ieee80211_find_sta(mac->vif, mac_addr);
 }
 
+static inline u32 calculate_bit_shift(u32 bitmask)
+{
+	if (WARN_ON_ONCE(!bitmask))
+		return 0;
+
+	return __ffs(bitmask);
+}
 #endif