wl3501_cs: 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}.

Include <net/cfg80211.h> for the defination of IEEE80211_BAND_2GHZ.
Because <net/cfg80211.h> includes <linux/ieee80211.h>, so we can
replace <linux/ieee80211.h> with <net/cfg80211.h>.

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

Cc: Arnaldo Carvalho de Melo <[e-mail]>
Signed-off-by: Zhao, Gang <[e-mail]>
Signed-off-by: John W. Linville <[e-mail]>

drivers/net/wireless/wl3501_cs.c

···
 #include <linux/slab.h>
 #include <linux/string.h>
 #include <linux/wireless.h>
-#include <linux/ieee80211.h>
+#include <net/cfg80211.h>
 
 #include <net/iw_handler.h>
 
···
 {
 	struct wl3501_card *this = netdev_priv(dev);
 
-	wrqu->freq.m = ieee80211_dsss_chan_to_freq(this->chan) * 100000;
+	wrqu->freq.m = 100000 *
+		ieee80211_channel_to_frequency(this->chan, IEEE80211_BAND_2GHZ);
 	wrqu->freq.e = 1;
 	return 0;
 }