[WATCHDOG] move platform probe and remove function to devinit and devexit

A pointer to probe and remove functions is passed to the core via
platform_driver_register and so the function must not disappear when the
.init sections are discarded.  Otherwise (if also having HOTPLUG=y)
unbinding and binding a device to the driver via sysfs will result in an
oops as does a device being registered late.

Signed-off-by: Wim Van Sebroeck <wim@iguana.be>