rtlwifi: Fix VHT NSS in RC

NSS is a argument of highest rate in RC, and it occupies bit 4-7 so use
ieee80211_rate_set_vht() to fill the values. Since it got correct rate
index, we don't need to check chips to assign NSS in set function anymore.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>