korad-kaxxxxp: Fix max. possible current for all devices.

Even though the devices/websites/manuals usually say 0..30V, the
hardware actually accepts up to 31V, both via serial as well as by
simply rotating the knob on the device (and our driver already
reflects that).

The same is true for current, it's usually 0..5A as per docs, but many
(probably all) devices accept 5.1A via serial and knob.

Thus, set the max current of all devices to 5.1A (or 3.1A for 3A
devices). We're assuming they all have this property, and we've seen
this in practice on at least three different versions of the device.