cfg80211: remove unused wiphy argument from cfg80211_wext_freq()

cfg80211_wext_freq() is declared in wext-compat.h, but its
parameter struct wiphy's declaration is not included there.
As the parameter isn't used, just remove it.

Signed-off-by: Zhao, Gang <gamerh2o@gmail.com>
[remove parameter instead of changing to netdev]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>