generic: routerbootpart.c: 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>

target/linux/generic/files/drivers/mtd/parsers/routerbootpart.c

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0
+// SPDX-License-Identifier: GPL-2.0-only
 /*
  * Parser for MikroTik RouterBoot partitions.
  *