ramips: watchdog: use maximum possible timeout by default

As the userspace has no means to determine the maximum possible timeout, use
that as the default and let the userspace lower it when necessary.

As the result the usual OpenWrt install (with busybox's watchdog trying to set
the timeout to 60s on start) is using a 33s timeout on an RT3052 clocked at
384MHz instead of the current 20s default.

Signed-off-by: Paul Fertser <fercerpav@gmail.com>

SVN-Revision: 33144