Bluetooth: Host: Non-discardable event buffer size is 255 for ext adv

After https://github.com/zephyrproject-rtos/zephyr/pull/41337

, extended
advertising reports are no longer discardable. Ensure the
non-discardable events are large enough.

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>