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>

drivers/staging/greybus/core.c

@@ -272,5 +272,5 @@ static void __exit gb_exit(void)
 }
 module_exit(gb_exit);
 
-MODULE_LICENSE("GPL");
+MODULE_LICENSE("GPL v2");
 MODULE_AUTHOR("Greg Kroah-Hartman <gregkh@linuxfoundation.org>");

drivers/staging/greybus/es1.c

@@ -694,5 +694,5 @@ static struct usb_driver es1_ap_driver = {
 
 module_usb_driver(es1_ap_driver);
 
-MODULE_LICENSE("GPL");
+MODULE_LICENSE("GPL v2");
 MODULE_AUTHOR("Greg Kroah-Hartman <gregkh@linuxfoundation.org>");

drivers/staging/greybus/es2.c

@@ -694,5 +694,5 @@ static struct usb_driver es1_ap_driver = {
 
 module_usb_driver(es1_ap_driver);
 
-MODULE_LICENSE("GPL");
+MODULE_LICENSE("GPL v2");
 MODULE_AUTHOR("Greg Kroah-Hartman <gregkh@linuxfoundation.org>");

drivers/staging/greybus/gpb.c

@@ -87,4 +87,4 @@ static void __exit gpbridge_exit(void)
 }
 module_exit(gpbridge_exit);
 
-MODULE_LICENSE("GPL");
+MODULE_LICENSE("GPL v2");