staging:iio:ad2s90: Replace license text w/ SPDX identifier

This patch removes the license boilerplate text at the top of ad2s90.c
and, instead, adds the SPDX GPL-2.0 license identifier, which solves the
checkpatch.pl warning:
"WARNING: Missing or malformed SPDX-License-Identifier tag in line 1".

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Acked-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>