ieee80211: remove function ieee80211_{dsss_chan_to_freq, freq_to_dsss_chan}

Function ieee80211_{dsss_chan_to_freq, freq_to_dsss_chan} have been
replaced with ieee80211_{channel_to_frequency, frequency_to_channel}.

There should be no users of the two functions now. So remove them.

Cc: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Zhao, Gang <gamerh2o@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>