watchdog: dw_wdt: Try to get a 30 second watchdog by default

The dw_wdt_set_top() function takes in a value in seconds.  In
dw_wdt_open() we were calling it with a value that's supposed to
represent the maximum value programmed into the "top" register with a
comment saying that we were trying to set the watchdog to its maximum
value.  Instead we ended up setting the watchdog to ~15 seconds.

Let's fix this.  However, setting things to the "max" gives me an 86
second watchdog in the system I'm looking at.  86 seconds feels a
little too long.  We'll explicitly choose 30 seconds as a more
reasonable value.

NOTE: Ideally this driver should be transitioned to be a real watchdog
driver.  Then we could use "watchdog_init_timeout" and let the timeout
be specified in a number of ways (device tree, module parameter, etc).
This patch should be considered a bit of a stopgap solution.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>