Bluetooth: controller: Enable CIS establishment in same ACL instant

If CIS offset in LL_CIS_IND is less than EVENT_OVERHEAD_START_US, the
controller is not able to establish the CIS in the ACL connection event
specified, but needs to start setup one connection event earlier.

With this commit, if offset is larger than EVENT_OVERHEAD_START_US the
first CIS event can be prepared in due time immediately.

This enables the controller to setup first CIS, even when instant is
equal to the ACL event_counter.

Signed-off-by: Morten Priess <mtpr@oticon.com>