watchdog: mpc8xxx: simplify registration

Since commit ef90174f ("watchdog: watchdog_core: Add watchdog
registration deferral mechanism") there is no need to delay the call to
watchdog_register_device any more. So simplify the registration code
accordingly.

Resetting wd_base to NULL can the also be dropped because nothing
depends on it being NULL to signal probe failure any more. (The matching
wd_base = NULL in .remove was missing, too.)

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>