Bluetooth: Controller: Fix extended scan rx flush for continous scan

Fix missing progression of the rx_last pointer when
appending rx buffers before flushing them towards Host.
Under continuous scanning, as the disabled_cb would only
be called when reference count reaches zero, the rx_last
pointer needs to progress when appending the rx buffers.

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