nl80211: make chandef parser take nlattr arg

The chandef parsing function doesn't need to
access genl_info. It just needs the attrs field.

This makes it easier for further reuse.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
(cherry picked from http://mid.gmane.org/1390226968-18928-2-git-send-email-michal.kazior@tieto.com)