Bluetooth: Controller: Fix incorrect CIS offset accepting peer instant

Fix incorrect CIS offset in use if instant is picked from
the peer sent CIS RSP PDU. Instead, keep the instant that
was sent in the CIS REQ PDU as the instant to send in the
CIS IND PDU.

This fixes CIS failed to be established when dissimilar
ACL and ISO intervals are in use.

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