wdt_i6300esb: register a reset function

The device shall set its default hardware state after each reset.
This includes that the timer is stopped which is especially important
if the guest does a reboot independantly of a watchdog bite. I moved
the initialization of the state variables completely from the init
to the reset function which is called right after init during the
first boot and afterwards during each reboot.

Signed-off-by: Bernhard Kohl <bernhard.kohl@nsn.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>