Bluetooth: Controller: Fix Central CIS offset for dissimilar intervals

Fix Central CIS offset calculation for dissimilar ACL and
ISO intervals in use.

Mayfly execution of `mfy_cig_offset_get()` could be after
"LLL Prepare" or before depending on whether a previous radio
event is being preempted or not, respectively; the
`conn->lll.event_counter` may not be pre-incremented.
This race condition is fixed by the fact that we use a
constant instant delta value now.

Dissimilar ACL and ISO intervals may lead to ACL overlapping
or be too close to ISO event, causing preemption; under this
case ACLs "LLL Prepare" would run after `mfy_cig_offset_get`
causing incorrect calculation of CIS offset without the fix.

Remove redundant `instant` member in `ll_conn_iso_stream`
structure as a constant CIS Create instant delta is now
used.

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