rdtech-rd: Fix <inf> samples for RD6006

[DidierA]

On my RD6006, no current or voltage value where available: they all returned inf.
I tracked the bug to the current and voltage multipliers being 0, because they were not initialized.
This was due to the test in rdtech_dps_update_range for multiple ranges, which returns false in this case, causing the function to return without calling rdtech_dps_update_multipliers.

With this patch, the multipliers are initialized, and both sigrok-cli and smuview obtain the correct values.

The devc->model->n_ranges <= 1 test had been introduced with PR #205 to support RD6012P. I do not own any other RD6xxx power supply, so I could only test this with RD6006.

Example run:
Before this fix:

$ sigrok-cli -d rdtech-rd:conn=/dev/ttyUSB1 --samples 1
FRAME-BEGIN
V: inf V DC
I: inf mA DC
P: 11.93 W
FRAME-END

After this fix:

$ sigrok-cli -d rdtech-rd:conn=/dev/ttyUSB1 --samples 1
FRAME-BEGIN
V: 17.27 V DC
I: 692 mA DC
P: 11.95 W
FRAME-END

[DidierA]

@mdjurfeldt I would appreciate feedback on this, does this fix seem appropriate? Can you test on your RD6012P if it does not break ?
Than you.

[mdjurfeldt]

@DidierA Hi, looking at the code, I don't think it should break. I can try to test it during the coming few days.