Bluetooth: Controller: Fix window offset calculation

Fix window offset calculation which is missing the radio
rx ready delay timing be subtracted because the connection
offset from the start of the radio for scan window to the
end of the ADV_IND PDU includes the radio rx ready delay
timing, where as the determined window offset by advanced
scheduling calculates the offset from the ticks_at_expire
of the scan window to an active central connection's
ticks_at_expire.

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