greybus: MODULE_LICENSE cleanup

These are all GPLv2-only kernel modules, so properly set the correct
MODULE_LICENSE string to make static checkers happy.

Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
diff --git a/drivers/staging/greybus/gpb.c b/drivers/staging/greybus/gpb.c
index 33bbe79..d5747b2 100644
--- a/drivers/staging/greybus/gpb.c
+++ b/drivers/staging/greybus/gpb.c
@@ -87,4 +87,4 @@
 }
 module_exit(gpbridge_exit);
 
-MODULE_LICENSE("GPL");
+MODULE_LICENSE("GPL v2");