watchdog: iTCO_wdt: Export vendorsupport

In preparation for making ->smi_res optional the iTCO_wdt driver needs
to know whether vendorsupport is being set to non-zero. For this reason
export the variable.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>