mac80211: fix internal scan request

The internal scan request mac80211 uses to
scan for IBSS networks was set up to contain
no channels at all because n_channels wasn't
set after setting up the channels array. Fix
this to properly scan for networks.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>