Bluetooth: Controller: Fix missing PDU next pointer initialization

When supporting chain PDUs, after HCI reset the first PDU
allocated did not initialize its next pointer causing use
of unallocated PDUs for chaining while a duplicate of such
PDU buffer being allocated for other states/roles. This
causes in certain cases both Extended and Periodic PDUs
having same data, the Extended Advertising PDU being
transmitted at the instant of the Periodic Advertising.

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