rapidio: update for destination ID allocation

Address comments provided by Andrew Morton:
https://lkml.org/lkml/2012/10/3/550



 - Keeps consistent kerneldoc compatible comments style for new static
   functions.
 - Removes unnecessary complexity from destination ID allocation
   routine.
 - Uses kcalloc() for code clarity.

Signed-off-by: Alexandre Bounine <alexandre.bounine@idt.com>
Cc: Matt Porter <mporter@kernel.crashing.org>
Cc: Li Yang <leoli@freescale.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>