Bluetooth: don't assume key size is 16 when the command fails

With this change, the encryption key size is not assumed to be 16 if the
read_encryption_key_size command fails for any reason.  This ensures
that if the controller fails the command for any reason that the
encryption key size isn't implicitely set to 16 and instead take a more
concervative posture to assume it is 0.

Signed-off-by: Alain Michaud <alainm@chromium.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>