iio: Declare event_attrs field of iio_info structure as const

The event_attrs field of iio_info structure is only initialized once
whenever an object of iio_info is created. After that this field
is never modified again anywhere in the kernel. So, declare event_attrs
field of iio_info as a const struct attribute_group.
Checked for occurences throughout the kernel using grep and
coccinelle.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>