iio: accel: bma180: Use explicit member assignment

This uses the C99 explicit .member assignment for the
variant data in struct bma180_part_info. This makes it
easier to understand and add new variants.

Cc: Peter Meerwald <pmeerw@pmeerw.net>
Cc: Oleksandr Kravchenko <o.v.kravchenko@globallogic.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>