iio: adc: ti-ads7950: Change regulator matching string to "vref"

This changes the reference voltage regulator matching string from "refin"
to "vref". This is to be consistent with other A/DC chips that also use
"vref-supply" in their device tree bindings.

Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>