mac80211: use constant time comparison with keys

Otherwise we risk leaking information via timing side channel.

Change-Id: I777afd16f7b0ca8a5ab1b95ddc66689afdc96636
Fixes: fdf7cb4185b6 ("mac80211: accept key reinstall without changing anything")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>