ipmi: Reset response handler when failing to send the command

When failing to send a command we don't expect a response. Clear the
`null_user_handler` like is done in the success path.

Signed-off-by: Markus Boehme <markubo@amazon.com>
Message-Id: <1599495937-10654-1-git-send-email-markubo@amazon.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>