mac80211: introduce hw config change flags

This makes mac80211 notify the driver which configuration
actually changed, e.g. channel etc.

No driver changes, this is just plumbing, driver authors are
expected to act on this if they want to.

Also remove the HW CONFIG debug printk, it's incorrect, often
we configure something else.

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