generic: platform/mikrotik: disambiguate SPDX-License-Identifier

I meant it to be GPL-2.0-only, as evidenced by the boilerplate.

Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>
(cherry picked from commit 6934b20912d308854b7adf63a7098d38bf67d153)

target/linux/generic/files/drivers/platform/mikrotik/rb_hardconfig.c

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0
+// SPDX-License-Identifier: GPL-2.0-only
 /*
  * Driver for MikroTik RouterBoot hard config.
  *

target/linux/generic/files/drivers/platform/mikrotik/routerboot.c

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0
+// SPDX-License-Identifier: GPL-2.0-only
 /*
  * Driver for MikroTik RouterBoot flash data. Common routines.
  *

target/linux/generic/files/drivers/platform/mikrotik/routerboot.h

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0
+// SPDX-License-Identifier: GPL-2.0-only
 /*
  * Common definitions for MikroTik RouterBoot data.
  *