Bluetooth: controller: split: Fix DLE during data transmission

Fix bug in Data Length Update procedure that caused the
connection to drop due to the implementation sending bigger
PDU before the peer has acknowledged the receipt of Length
Response PDU.

Relates to commit ffbbec7a89ca ("Bluetooth: controller: Fix
DLE during data transmission").

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