Bluetooth: hci_event: Fix using memcmp when comparing keys

memcmp is not consider safe to use with cryptographic secrets:

 'Do  not  use memcmp() to compare security critical data, such as
 cryptographic secrets, because the required CPU time depends on the
 number of equal bytes.'

While usage of memcmp for ZERO_KEY may not be considered a security
critical data, it can lead to more usage of memcmp with pairing keys
which could introduce more security problems.

Fixes: 455c2ff0 ("Bluetooth: Fix BR/EDR out-of-band pairing with only initiator data")
Fixes: 33155c4a ("Bluetooth: hci_event: Ignore NULL link key")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>