iio: adc: vf610_adc: Fix VALT selection value for REFSEL bits

commit d466d3c1217406b14b834335b5b4b33c0d45bd09 upstream.

In order to select the alternate voltage reference pair (VALTH/VALTL), the
right value for the REFSEL field in the ADCx_CFG register is "01", leading
to 0x800 as register mask. See section 8.2.6.4 in the reference manual[1].

[1] http://www.nxp.com/docs/en/reference-manual/VFXXXRM.pdf



Fixes: a7754276 ("iio:adc:imx: add Freescale Vybrid vf610 adc driver")
Signed-off-by: Stefan-Gabriel Mirea <stefan-gabriel.mirea@nxp.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>