staging: wlan-ng: replace function ieee80211_dsss_chan_to_freq()

Replace ieee80211_dsss_chan_to_freq() with more generic
ieee80211_channel_to_frequency(), and add a variable to deal with 80
characters problem.

File cfg80211.c is included by p80211netdev.c, p80211netdev.c includes
<net/cfg80211.h>, both ieee80211_channel_to_frequency() and
IEEE80211_BAND_2GHZ is defined / declared in <net/cfg80211.h>. So this
change is safe.

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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>