Bluetooth: Controller: Move aux sync scanning to lll_sync

Periodic advertising train scanner implemented in lll_scan_aux adds lots
of branches that cannot be compiled out with periodic advertising sync
disabled.

This commit moves sync parts of the code from lll_scan_aux to lll_sync.

Signed-off-by: Andrzej Kaczmarek <andrzej.kaczmarek@codecoup.pl>