Bluetooth: Controller: Fix to use non-discardable buffer

Fix to use non-discardable Rx buffer for handling Extended
and Periodic Advertising Report.

Regression introduced in
commit 18171bc7 ("Bluetooth: Host: Non-discardable event
buffer size is 255 for ext adv"). The default discardable Rx
buffer size was reduced causing increased fragmentation and
deadlock trying to get more free discardable Rx buffers to
generate HCI Extended and Periodic Advertising reports.

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