Bluetooth: controller: update k_work_pending to new API

k_work_pending is now called k_work_is_pending.

These two uses are correct assuming the functions that invoke them are
not involved in race conditions: that the work is not pending is an
allowed condition for modifying state that will be used by the work
handler.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>