i2c: slave: use SPDX license information

SPDX license helps developers and machines to know the license governing a particular file hence easing work

---
 drivers/i2c/i2c-core-slave.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 6f56e65..15c7f7a 100644
@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0+
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Linux I2C core slave support code
  *

2.7.4