mac80211: use a counter for remain-on-channel cookie

Instead of using the pointer which can be re-used
fairly quickly due to allocator patterns and then
makes debugging difficult, maintain a counter and
use its value. Since it's a 64-bit value it can't
really wrap, but catch that case anyway since it
most likely points to a bug somewhere.

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