iio: Mark iio_device_type as const

The iio_device_type struct is never modified, mark it as const. This allows
it to be placed in a read-only memory section, which will protect against
accidental or deliberate modification.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20211031080421.2086-1-lars@metafoo.de


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>