Bluetooth: Controller: Fix LE Ping PDU dispatch

It was observed that due to possible CRC errors, one
connection interval was not sufficient by the peer to
respond to LE Ping PDU which caused the Controller to
generate the Authenticated Payload Timeout event to host.

This fix advanced the dispatch of LE Ping PDU by 6
connection intervals that the peer would listen to before
the 30s timeout.

Change-id: I6c292c623047a05b4e771e70093d87228db62cce
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>