samples: Bluetooth: df: Fix too early per sync create timeout

Periodic advertising synchronization create had a timeout set
to fixed value of 10 seconds. BT 5.3 Core specification defines
synchronization timeout as 6 consecutive periodic advertising
events. When advertiser set the periodic interval to be more than
1.6 second it was possible the application timeout is reached
before time allowed by BT Core specification.

Changed implementation of timeout to depend on the periodic
advertising interval.

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