Bluetooth: Controller: Fix multiple Extended Adv chain reception

Fix assertion when enabling simultaneous multiple Extended
Advertising chain reception that is enabled by increasing
supported auxiliary scan contexts.

ULL sets the association of aux context to scan and sync
context, and LLL resets the association; this is safer
compared to earlier implementation where ULL did both the
association and reset which caused aux context memory
leak.

Supported auxiliary scan contexts can be increased using
CONFIG_BT_CTLR_SCAN_AUX_SET value.

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