rtlwifi: rtl8192ce: replace _rtl92c_evm_db_to_percentage with generic version

Function _rtl92c_evm_db_to_percentage is functionally identical
to the generic version rtl_evm_db_to_percentage, so remove
_rtl92c_evm_db_to_percentage and use the generic version instead.

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