mac80211: automatic IBSS channel selection

When joining an ad-hoc network, the user is currently required to specify
the channel. The network will not be joined otherwise, unless it happens
to be sitting on the currently active channel.

This patch implements automatic channel selection when the user has not
locked the interface onto a specific channel.

Signed-off-by: Daniel Drake <dsd@gentoo.org>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>