staging: iio: ad7780: remove unnecessary stashed voltage value

This patch removes the unnecessary field int_vref_mv in ad7780_state
referring to the device's voltage.

Signed-off-by: Renato Lui Geh <renatogeh@gmail.com>
Acked-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>