Bluetooth: controller: Fix connection failed to be established

Fix connection failed to be established regression
introduced by the commit 350c569a ("Bluetooth:
controller: Avoid offseting to lldata").

As the Rx-ed PDU buffer is re-used to construct the
connection complete message towards HCI, the fields in the
Rx-ed PDU need to be backup for future use in the control
path. Here the channel selection bit is backup now.

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