iio: chemical: vz89x hardware i2c workaround

Chipset sometime updates in the middle of a reading causing it
to reset the data pointer, and causing invalid reading of previous data.

We can check for this invalid state by reading MSB of the resistance
reading that is always zero, and by also confirming the VOC_short isn't
zero.

Signed-off-by: Matt Ranostay <mranostay@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>