IPMI: fix comparison in demangle_device_id

Coverity spotted some incorrect code in a recent change to the IPMI driver;
this patch make sure the data is really long enough to pull the
manufacturer id and product id out of a get device id message.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Cc: Adrian Bunk <bunk@kernel.org>
Cc: Stian Jordet <liste@jordet.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>