Bluetooth: Controller: Fix CTE req periodically after req is disabled

There is an error in the condition that checks if new CTE_REQ should
be started after end of connection event. The condition verifies if
counter req_expire is set to zero. Then new CTE_REQ is started
irrespectively to CTE_REQ being disabled.

req_interval is used to store information if the CTE_REQ is:
- periodic, then value doesn't equal zero,
- single shot or disabled, then value equals zero.

The condition should verify if the req_interval is not zero and
req_expire is not zero. The second part of the if condition is
required to avoid starting next CTE_REQ until last one has been
completed.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>