iio: mxs-lradc: fix iio channel map regression

Since commit c8231a9af8147f8a ("iio: mxs-lradc: compute temperature
from channel 8 and 9") with the removal of adc channel 9 there is
no 1-1 mapping in the channel spec.

All hwmon channel values above 9 are accessible via there index minus
one. So add a hidden iio channel 9 to fix this issue.

Signed-off-by: Stefan Wahren <[e-mail]>
Acked-by: Alexandre Belloni <[e-mail]>
Reviewed-by: Marek Vasut <[e-mail]>
Cc: [e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/staging/iio/adc/mxs-lradc.c

···
 		.channel = 8,
 		.scan_type = {.sign = 'u', .realbits = 18, .storagebits = 32,},
 	},
+	/* Hidden channel to keep indexes */
+	{
+		.type = IIO_TEMP,
+		.indexed = 1,
+		.scan_index = -1,
+		.channel = 9,
+	},
 	MXS_ADC_CHAN(10, IIO_VOLTAGE),	/* VDDIO */
 	MXS_ADC_CHAN(11, IIO_VOLTAGE),	/* VTH */
 	MXS_ADC_CHAN(12, IIO_VOLTAGE),	/* VDDA */