ipmi:ssif: Fix a shutdown race

It was possible for the SSIF thread to stop and quit before the
kthread_stop() call because ssif->stopping was set before the
stop.  So only exit the SSIF thread is kthread_should_stop()
returns true.

There is no need to wake the thread, as the wait will be interrupted
by kthread_stop().

Signed-off-by: Corey Minyard <cminyard@mvista.com>