Bluetooth: controller: split: Fix slave latency enable

Fix regression in porting slave latency, slave latency
enabled was not used, cause slave latency to be applied
before first packet being acknowledged by the master.

Regression in commit 5dff214d ("Bluetooth: controller:
split: Fix missing slave latency impl.").

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