This is a flag from the MAC that says the received packet didn't match
a keycache slot. This isn't technically a problem as WEP keys don't
match keycache slots (they're "global" keys), but it could be useful
for tracking down CCMP decryption failures.
Right now it's a no-op - it mirrors what the AR9300 HAL does and it
just increments a counter. But, hey, maybe one day I'll use it for
diagnosing keycache/CCMP decrypt issues.
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