Fix the watchdog timeout calculation to prevent wrap. The RPi hardware

Fix the watchdog timeout calculation to prevent wrap.  The RPi hardware
can't do a timeout bigger than 15 seconds.  The code wasn't checking for
this and because bitmasking was involved the requested timeout was
basically adjusted modulo-16.  That led to things like a 128 second
timeout actually being a 9 second timeout, which accidentally worked fine
until watchdogd was changed to only pet the dog once every 10 seconds.