wifi: rtlwifi: rtl8821ae: phy: fix an undefined bitwise shift behavior

Clang static checker warns:

drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c:184:49:
	The result of the left shift is undefined due to shifting by '32',
	which is greater or equal to the width of type 'u32'.
	[core.UndefinedBinaryOperatorResult]

If the value of the right operand is negative or is greater than or
equal to the width of the promoted left operand, the behavior is
undefined.[1][2]

For example, when using different gcc's compilation optimization options
(-O0 or -O2), the result of '(u32)data << 32' is different. One is 0, the
other is old value of data. Let _rtl8821ae_phy_calculate_bit_shift()'s
return value less than 32 to fix this problem. Warn if bitmask is zero.

[1] https://stackoverflow.com/questions/11270492/what-does-the-c-standard-say-about-bitshifting-more-bits-than-the-width-of-type
[2] https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf

Fixes: 21e4b072 ("rtlwifi: rtl8821ae: Move driver from staging to regular tree")
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://lore.kernel.org/r/20231127013511.26694-2-suhui@nfschina.com

drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c

@@ -29,9 +29,10 @@ static void _rtl8821ae_phy_rf_serial_write(struct ieee80211_hw *hw,
 					   u32 data);
 static u32 _rtl8821ae_phy_calculate_bit_shift(u32 bitmask)
 {
-	u32 i = ffs(bitmask);
+	if (WARN_ON_ONCE(!bitmask))
+		return 0;
 
-	return i ? i - 1 : 32;
+	return __ffs(bitmask);
 }
 static bool _rtl8821ae_phy_bb8821a_config_parafile(struct ieee80211_hw *hw);
 /*static bool _rtl8812ae_phy_config_mac_with_headerfile(struct ieee80211_hw *hw);*/