cappuccino: remove unused op_mfspr wire
cappuccino/ctrl: default spr acks to 1 instead of 0

If the ack is to be used for mfspr this is necessary
for reading out 0 as default, otherwise the cpu will lock-up
while waiting for the ack.
cappuccino/ctrl: avoid taking exceptions when bubble is in ctrl stage

since the bubble isn't a real instruction, the normal rules
for what has been executed isn't exactly correct when the
bubble has entered control stage, so we have to wait until
a real instruction has advanced into ctrl stage before taking
the exception.