ipmi:ssif: Add missing unlock in error branch

When getting flags, a response to a different message would
result in a deadlock because of a missing unlock.  Add that
unlock and a comment.  Found by static analysis.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: stable@vger.kernel.org # 3.19
Signed-off-by: Corey Minyard <cminyard@mvista.com>