firmware: switch the DAC off too when switching off Vio

[electroniceel]

Without this, the DAC still outputs it's control voltage even if Vio is off. This goes through the feedback resistors to the output. I measured about 15 mV without this patch on a port that was previously enabled. With this patch it properly goes down to zero.

While such a low voltage usually shouldn't be a problem, I consider it unclean design, especially if it is so easy to fix.

I based this off the wip-revC2-firmware branch to avoid a merge conflict with the firmware ihex. All firmware changes also have to contain the binary firmware. So this is intended to be merged after wip-revC2-firmware was merged.

[electroniceel]

I will update this once #242 is merged.

[whitequark]

@electroniceel #242 is merged, could you take a look please?