watchdog: w83627hf: Enable watchdog device only if not already enabled

There is no need to enable the watchdog device if it is already enabled.
Also, when enabling the watchdog device, only set the watchdog device
enable bit and do not touch other bits; depending on the chip type,
those bits may enable other functionality.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>