gpio: amd-fch: Fix bogus SPDX identifier

spdxcheck.py complains:

 include/linux/platform_data/gpio/gpio-amd-fch.h: 1:28 Invalid License ID: GPL+

which is correct because GPL+ is not a valid identifier. Of course this
could have been caught by checkpatch.pl _before_ submitting or merging the
patch.

 WARNING: 'SPDX-License-Identifier: GPL+ */' is not supported in LICENSES/...
 #271: FILE: include/linux/platform_data/gpio/gpio-amd-fch.h:1:
 +/* SPDX-License-Identifier: GPL+ */

Fix it under the assumption that the author meant GPL-2.0+, which makes
sense as the corresponding C file is using that identifier.

Fixes: e09d168f ("gpio: AMD G-Series PCH gpio driver")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>

include/linux/platform_data/gpio/gpio-amd-fch.h

-/* SPDX-License-Identifier: GPL+ */
+/* SPDX-License-Identifier: GPL-2.0+ */
 
 /*
  * AMD FCH gpio driver platform-data