rndis_wlan: replace function ieee80211_{dsss_chan_to_freq, freq_to_dsss_chan}

Replace ieee80211_{dsss_chan_to_freq, freq_to_dsss_chan} with more
generic ieee80211_{channel_to_frequency, frequency_to_channel}.

File <net/cfg80211.h> has already been included. It's safe to use
IEEE80211_BAND_2GHZ here.

This change is a preparation for the removal of function
ieee80211_{dsss_chan_to_freq, freq_to_dsss_chan}.

Cc: Jussi Kivilinna <jussi.kivilinna@iki.fi>
Signed-off-by: Zhao, Gang <gamerh2o@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rndis_wlan.c

@@ -1290,7 +1290,8 @@ static int set_channel(struct usbnet *usbdev, int channel)
 	if (is_associated(usbdev))
 		return 0;
 
-	dsconfig = ieee80211_dsss_chan_to_freq(channel) * 1000;
+	dsconfig = 1000 *
+		ieee80211_channel_to_frequency(channel, IEEE80211_BAND_2GHZ);
 
 	len = sizeof(config);
 	ret = rndis_query_oid(usbdev,