Bluetooth: controller: Fix uninitialized rl_idx value

Fix uninitialized rl_idx value for the Extended Advertising
PDU received in the auxiliary channels. This caused
uninitialized rl_idx to be used by HCI layer and fail an
assert check in ull_filter.c file.

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