watchdog: ath79_wdt: switch to clk_prepare/clk_disable

Replace clk_enable() and clk_disable() calls with
clk_prepare_enable() and clk_disable_unprepare()
to get ready for the migration to the common clock
framework.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>