watchdog: mpc8xxx_wdt: fix build

Since 1c48a5c9 (dt: Eliminate of_platform_{,un}register_driver)
mpc8xxx_wdt no longer builds as it tries to refer to a 'match' variable
rather than ofdev->dev.of_match that it checks just before.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>