rtlwifi: constify rate_control_ops structure

rate_control_ops structure is only passed as an argument to the
function ieee80211_rate_control_{register/unregister}. This argument
is of type const, so declare the structure as const.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>