Bluetooth: Enable LE encryption events only when supported

The support for LE encryption is optional. When encryption is not
supported then also do not enable the encryption related events.

This moves the event mask setting to the third initialization
stage to ensure that the LE features are available.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>