Bluetooth: controller: Be fair when pre-empting a ticker

When force scheduling a ticker use the number of times the
tickers have already skipped their intervals to decide if
the forced ticker can pre-empt the colliding ticker. This
introduces a fairness amongst tickers contesting for the
overlapping time slice.

Flashing in co-operation with Radio needs to be fair in
order to avoid connection supervision timeouts.

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