watchdog: i6300esb: do not hardcode heartbeat limits

The minimum, maximum and default values for the watchdog heartbeat
(timeout) were hardcoded in several places (including module parameter
description and warning message for invalid module parameter value).

This patch adds macros for the aforementioned values and replaces all
occurences of hardcoded values by these macros.

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>