Bluetooth: Controller: Fix cis_offset_min to have additional margin

As the coarse tick of ACL prepare and CIG prepare jitter by
+/- 1 tick, hence when initially scheduling the CIG place it
with enough margin so that cis_offset_min calculated when
there already exists a CIG can be larger than peer's
cis_offset_min being negotiated.

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