uio: add SPDX license tag

Replace short statement in comment with proper SPDX
license tag.
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/uio/uio.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * drivers/uio/uio.c
  *
@@ -9,8 +10,6 @@
  * Userspace IO
  *
  * Base Functions
- *
- * Licensed under the GPLv2 only.
  */
 
 #include <linux/module.h>