Bluetooth: Controller: Fix connection update interval_us variables

Fix connection update microsecond interval variable data
type, to use 32-bit so that a value upto 2000 seconds, i.e.
4 seconds interval and 499 peripheral latency can be stored.

Regression in commit abfe5f17 ("Bluetooth: Controller:
1 ms connection").

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