nand: create device with 0640 permission

rw-r----- is the usual/standard permissions for such devices.

Direct commit to stable/12 as this code has been removed in HEAD.

MFC after:	1 week
Sponsored by:	The FreeBSD Foundation

sys/dev/nand/nand_cdev.c

@@ -101,7 +101,7 @@ nand_make_dev(struct nand_chip *chip)
 	parent_unit = device_get_unit(parent);
 	unit = parent_unit * 4 + chip->num;
 	chip->cdev = make_dev(&nand_cdevsw, unit, UID_ROOT, GID_WHEEL,
-	    0666, "%s%d.%d", name, parent_unit, chip->num);
+	    0640, "%s%d.%d", name, parent_unit, chip->num);
 
 	if (chip->cdev == NULL)
 		return (ENXIO);