cfg80211: move cookie_counter out of wiphy

There's no reason for drivers to be able to access the
cfg80211 internal cookie counter; move it out of the
wiphy into the rdev structure.

While at it, also make it never assign 0 as a cookie
(we consider that invalid in some places), and warn if
we manage to do that for some reason (wrapping is not
likely to happen with a u64.)

Signed-off-by: Johannes Berg <johannes.berg@intel.com>