ipmi_devintf: Small lock rework

The mutex didn't really serve any useful purpose, from what I can
tell, and it would just get in the way.  So remove it.

Removing that required a mutex around the default value setting and
getting, so just use the receive mutex for that.

Also pull the fasync stuff outside of the lock for adding the data
to the queue, since it didn't need to be there.

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