Bluetooth: controller: Fix connection update supervision timeout

In the commit dd52b8ea ("Bluetooth: controller: Fix
first connection interval timing"), instead of using just a
tick unit as workaround, microseconds corresponding to a
tick unit was used while calculating the window offset to be
used at the connection update instant. This introduced an
error in scheduling the first event with new connection
parameters, causing supervision timeout of connection update
procedure.

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