Bluetooth: Controller: Fix connection update window offset data type

Fix regression in refactored LLCP using uint16_t instead of
uint32_t in storing the win_offset_us value. This caused
connection update to fail with incorrect window offset being
used to schedule the connection radio events.

Regression since commit e1c2c36f ("Bluetooth: controller:
llcp: set refactored as default").

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>
(cherry picked from commit 2579690b)