iio: mma8452: replace license description with SPDX specifier

This replaces the custom license information text with the appropriate
SPDX identifier. While the information here stays the same, it is easier
to read.

Signed-off-by: Martin Kepplinger <martink@posteo.de>
Acked-by: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Acked-by: Harinath Nampally <harinath922@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>