rtlwifi: Remove extraneous argument for rate mapping

Four of the drivers (92ce, 92cu, 92de, and 92se) supply an argument to the
rate-mapping routine that is never used, thus it can be removed.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>