rtlwifi: rtl8192cu: use generic rtl_query_rxpwrpercentage

Function _rtl92c_query_rxpwrpercentage is identical to the generic
version rtl_query_rxpwrpercentage. Remove _rtl92c_query_rxpwrpercentage
and use the generic function.

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