rtlwifi: rtl8821ae: Place braces around empty if() body

Fixes the following W=1 kernel build warning(s):

 drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c: In function ‘_rtl8812ae_phy_get_txpower_limit’:
 drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c:2453:3: warning: suggest braces around empty body in an ‘if’ statement [-Wempty-body]

Cc: Ping-Ke Shih <pkshih@realtek.com>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Larry Finger <Larry.Finger@lwfinger.net>
Cc: linux-wireless@vger.kernel.org
Cc: netdev@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201102112410.1049272-35-lee.jones@linaro.org