watchdog: i6300esb: use the watchdog subsystem

Change the i6300esb driver to use the watchdog subsystem instead of the
legacy watchdog API. This is mainly just getting rid of the "write" and
"ioctl" methods and part of the watchdog control logic (which are all
implemented by the watchdog subsystem).

Even though the watchdog subsystem supports registering and handling
multiple watchdog devices at the same time, the i6300esb driver still
has a limitation of only one i6300esb device due to some global variable
usage that comes from the original design. However, the driver can now
coexist with other watchdog devices (supported by different drivers).
Signed-off-by: Radu Rendec <rrendec@arista.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>