iio: adc: ad7606: fix oversampling gpio array

gpiod_set_array_value was misused here: the implementation relied on the
assumption that an unsigned long was required for each gpio, while the
function expects a bit array stored in "as much unsigned long as needed
for storing one bit per GPIO", i.e it is using a bit field.

This leaded to incorrect parameter passed to gpiod_set_array_value, that
would set 1 value instead of 3.
It also prevents to select the software mode correctly for the AD7606B.

Fixes: d2a415c8 ("iio: adc: ad7606: Add support for AD7606B ADC")
Fixes: 41f71e5e ("staging: iio: adc: ad7606: Use find_closest() macro")
Signed-off-by: Guillaume Stols <gstols@baylibre.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>