Bluetooth: controller: Fix scheduling stall in BT_CTLR_LOW_LAT feature

Fix radio event scheduling stall when using BT_CTLR_LOW_LAT
feature.

Performing a Connection Update Procedure on an ACL
connection while simultaneously having continuous scanning
causes the ACL connection to drop in nRF51 series which uses
BT_CTLR_LOW_LAT by default.

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