ipmi:ssif: Fix handling of multi-part return messages

The block number was not being compared right, it was off by one
when checking the response.

Some statistics wouldn't be incremented properly in some cases.

Check to see if that middle-part messages always have 31 bytes of
data.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Cc: stable@vger.kernel.org # 4.4