rapidio: fix comment

The resource index for the mailboxes was incorrect.

Signed-off-by: Chad Reese <kreese@caviumnetworks.com>
Acked-by: Alexandre Bounine <alexandre.bounine@idt.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>