ipmi: fix platform return check

The wrong return value is being tested when allocating a platform device
in the IPMI SI code.  Check the right value.

Signed-off-by: Corey Minyard <minyard@acm.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>