rtlwifi: Install updated rate-mapping routine

In preparation for fixing the rate-mapping situation, place a driver-agnostic
version in rtlwifi. This one contains the updated rate incormation.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Chaoming Li <chaoming_li@realsil.com.cn>
Signed-off-by: John W. Linville <linville@tuxdriver.com>