ipmi: Fix return value when a message is truncated

The user can ask the message to be returned even if it didn't supply
enough memory for the data, and it will return an error but still
fills in as much data as possible.   However, the return value
wasn't being set to an error, it was being overwritten.  Create a
second return value for that case.

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