watchdog: improve seting watchdog timeout and frequency

watchdog: improve seting watchdog timeout and frequency

Due to the watchdog file descriptor check in both watchdog_timeout and
watchdog_frequency it's impossible to set the timeout/frequency via ubus
in case the watchdog was stopped.
Fix this by removing the watchdog file descriptor check in both functions
and by caching locally the set watchdog driver timeout value. The latter
will be used to set the watchdog driver timeout value in case the
watchdog is active or when the watchdog is started.
In addition when parsing the watchdog ubus call check if timeout
attribute is set so the correct value is used when doing the frequency
sanity check.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>