ipmi: fix return from atca_oem_poweroff_hook

A void returning function returned the return value of another void
returning function...

Spotted by sparse.

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