The i.MX21/i.MX6 watchdog cannot be disabled after it was activated.
Bit 2 (WDE) cannot be set to 0 again. Notify the user about that by
returning -ENOSYS.
Signed-off-by: Stefan Christ <s.christ@phytec.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>