gpio: restore original GPLv2+ license of gpiolib-of.c sources

It's easy to verify that the change of drivers/gpio/gpiolib-of.c license
header to SPDX standard changes the license from GPLv2+ to GPLv2, and
this change corrects it.

Fixes: dae5f0afcfc3 ("gpio: Use SPDX header for core library")
Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
diff --git a/drivers/gpio/gpiolib-of.c b/drivers/gpio/gpiolib-of.c
index 7f1260c..59cb873 100644
--- a/drivers/gpio/gpiolib-of.c
+++ b/drivers/gpio/gpiolib-of.c
@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0
+// SPDX-License-Identifier: GPL-2.0+
 /*
  * OF helpers for the GPIO API
  *