nimble/ll: Fix offset calculation in SyncInfo
nimble/ll: Fix offset calculation in SyncInfo

If advertising event is interleaved by current periodic event offset
was calculated from that event, and not future event. This resulted
in sending SyncInfo with offset=0 which was causing random failures
on LL/DDI/ADV/BV-56-C qualification test.

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble