Don't try to stop the IPMI watchdog timer if it is not running.

Don't try to stop the IPMI watchdog timer if it is not running.
Starting or stopping the IPMI watchdog is rather expensive with the
current implementation as all IPMI requests are bounced via thread.
This is not viable during shutdown or dumps, and this avoids headache
in the common case that the watchdog is not enabled.  The IPMI watchdog
should probably be reworked to not use a separate thread to fix this
in the case when the watchdog timer is enabled.

MFC after:	2 weeks