iwlwifi: mvm: LRU-assign key offsets

The current key offset assignment algorithm always uses the lowest
unused key offset, which will potentially lead to issues when the
firmware will change to take the key material for TX from the key
table rather than from the TX command.

In order to avoid those issues (and avoid forgetting about them)
change the key offset allocation algorithm now to avoid reusing key
offsets quickly.

The new algorithm always picks as the next offset the least recently
freed offset, i.e. the offset that has been unused for the longest
amount of time. This is implemented by having a generation counter
for each key offset that is incremented every time a key is deleted,
except for the one that's deleted, which is reset to zero. Thus the
highest counter is the key that's been unused longest.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>