Bluetooth: Controller: Fix missing connection handle invalidate

Fix missing connection handle invalidate on Controller
power up.

The connection context are zero-initialized on startup and
calls to `ll_connected_get()` would incorrectly return a
valid connection context pointer for connection handle 0.

Relates to commit fa02dc4d ("Bluetooth: Controller: Fix
missing reset of connection handle").

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>