iio: common: ssp_sensors: account for const type of of_device_id.data

This driver creates a number of const structures that it stores in the
data field of an of_device_id array.

Add const to the declaration of the location that receives a value
from the data field to ensure that the compiler will continue to check
that the value is not modified and remove the const-dropping cast on
the access to the data field.

Done using Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>