Implement `clear` for the softnpu ASIC.

[rcgoodfellow]

The dpd daemon expects that it can clear ASIC table state at startup. However, the softnpu TableOps implementation does not implement this:

dendrite/asic/src/softnpu/table.rs

Lines 104 to 107 in 2daa552

	fn clear(&self, _hdl: &Handle) -> AsicResult<()> {
	//TODO implement in softnpu
	Ok(())
	}

This can lead to unpredictable behavior as noted in oxidecomputer/maghemite#545.