verif/debug_controller: fix PLI dbg_get() call

dbg_get() doesn't seem to operate like a non-blocking assigment and this
was giving random failures where values hadn't been updated in the
expected order.  Guard against this by waiting for dbg_req to be set low
first before getting a new request.

Oddly this only failed on Icarus 0.9.7-1 but not
iverilog-0.9.20120609-3.fc20.x86_64.