wlan-ng: fixup staging driver for removal of ieee80211_dsss_chan_to_freq

Commit 3ebe8e257307 ("ieee80211: remove function
ieee80211_{dsss_chan_to_freq, freq_to_dsss_chan}") removed
ieee80211_dsss_chan_to_freq, but it neglected to account for this
staging driver...

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