iio: accel: mma8452: Constify static struct attribute_group

iio: accel: mma8452: Constify static struct attribute_group

The only usage of mma8452_event_attribute_group is to assign its address
to the event_attrs field in the iio_info struct, which is a const
pointer. Make it const to allow the compiler to put it in read-only
memory. This was the only non-const static struct in drivers/iio.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Link: https://lore.kernel.org/r/20200930232939.31131-1-rikard.falkeborn@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>