iio: adc: ads124: avoid buffer overflow

When initializing the priv->data array starting from index 1, there is one
less element to consider than when initializing the full array.

Fixes: e717f8c6

 ("iio: adc: Add the TI ads124s08 ADC code")
Signed-off-by: Vincent Stehlé <vincent.stehle@laposte.net>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Reviewed-by: Dan Murphy <dmurphy@ti.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>