Bluetooth: fix MGMT_EV_NEW_LONG_TERM_KEY event

This patch fixes how MGMT_EV_NEW_LONG_TERM_KEY event is build. Right now
val vield is filled with only 1 byte, instead of whole value. This bug
was introduced in
commit 1fc62c52

 ("Bluetooth: Fix exposing full value of shortened LTKs")

Before that patch, if you paired with device using bluetoothd using simple
pairing, and then restarted bluetoothd, you would be able to re-connect,
but device would fail to establish encryption and would terminate
connection. After this patch connecting after bluetoothd restart works
fine.

Signed-off-by: Jakub Pawlowski <jpawlowski@google.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>