watchdog: ath79_wdt: flush register writes

The watchdog register writes required to have a flush
in order to commit the values to the register. Without
the flush, the driver not function correctly on AR934X
SoCs.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Luis R. Rodriguez <mcgrof@qca.qualcomm.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>