Bluetooth: controller: Ignore connections from same peer

Ignore connection indications from peers that are already
connected. This is to bring the behavior of the controller in
accordance with [5.2, Vol 6, Part B, 4.5 Connection state]:
"If an advertiser receives a connection request from an initiator it
is already connected to, it shall ignore that request."

Signed-off-by: Wolfgang Puffitsch <wopu@demant.com>