cfg80211: fix docbook warning

When trying to generate documentation, at least xmldocs, we get the
following warning:

Warning(include/net/cfg80211.h:461): No description found for parameter 'nl80211_iftype'

Fix it by adding the iftype argument name to the
cfg80211_chandef_dfs_required() function declaration.

Reported-and-tested-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>