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

Function ieee80211_frequency_to_channel() will return 0 if frequency
is not correctly provided, so no need to set channel to 0 explicitly.

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}.
Signed-off-by: Zhao, Gang <gamerh2o@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>