rtlwifi: rtl8192de: use generic rtl_signal_scale_mapping

Function _rtl92de_signal_scale_mapping is identical to the generic
version rtl_signal_scale_mapping. Remove _rtl92de_signal_scale_mapping
and use the generic function.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>