[jtag,dv] Lots of tidyups in jtag_driver.sv

[rswarbrick]

The main impetus for this came from trying to sort out an rv_dm test which was failing because of incorrect use of a clocking block (see "Remove use of HOST_CB clocking block"), but I spent a while looking at the rather complicated code and trying to tidy it up a bit.

The commit described above should be the only one that changes behaviour (and it changes it for the better!). The others should all be tidyups that make the code clearer without changing behaviour.