MFC r297179: Optimize IPMI watchdog patting.

MFC r297179: Optimize IPMI watchdog patting.

Set watchdog timer parameters only when they really need to be changed.
In other cases just restart the timer with single Reset command instead
of two (Set and Reset).

From one side this visually reduces amount of CPU time burned in tight
loop waiting while some slow BMC configures its watchdog hardware, that
seems to be much more complicated task then just resetting the timer.

From another side on some BMCs those slow Set commands sometimes tend to
timeout, that leads to noisy log messages and even more CPU time burned,
so avoiding them can provide even bigger bonuses.

git-svn-id: svn://svn.freebsd.org/base/stable/10@297574 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f