Bluetooth: Controller: Fix missing ISO data packet receive

Fix missing ISO Data packet received by Synchronized
Receiver due to incorrect check on sink handle that did
not permit handle value of 0.

Fixed function to get ISO stream context to check for
valid ISO sync context, i.e. not being terminated.

Regression introduced in commit 7c89f1fe ("Bluetooth:
controller: Support for separate ISO RX data path").

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