ipmi: Allow an SMI sender to return an error

Getting ready for handling when a BMC is non-responsive or broken, allow
the sender operation to fail in an SMI.  If it was a user-generated
message it will return the error.

The powernv code was already doing this internally, but the way it was
written could result in deep stack descent if there were a lot of
messages queued.  Have its send return an error in this case.

Signed-off-by: Corey Minyard <corey@minyard.net>