rtlwifi: rtl8723be: avoid undefined behavior

Do not return undefined value for transmission power
if the rate is invalid.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>