watchdog: mpc8xxx: use better error code when watchdog cannot be enabled

checkpatch warns about ENOSYS, telling "ENOSYS means 'invalid syscall
nr' and nothing else". So use ENODEV instead.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>