Improve PL011 console interactivity

We are operating in "register mode" (not FIFO) which means if we clear an interrupt and have more data available, we need to re-raise the interrupt again.  Add two more cases where this is needed.
With this the interactivity on FreeBSD went to usable.  Before this after a certain event, it could take up to 15(?) additional charatcers to get previously pasted command lines echoed and executed.
I have a possible report about similar behaviour from people at ARM on Linux (uncofirmed) and I am putting the patch up so they can test.