rapidio: add handling of orphan port-write message

Add check for access to port-write (PW) message source device before
processing the PW message.  If source RIO device is not available (power
down or RIO link failure) trace back to a last available switch/port on
the PW message route and service failure at that point.

Signed-off-by: Alexandre Bounine <alexandre.bounine@idt.com>
Cc: Thomas Moll <thomas.moll@sysgo.com>
Cc: Matt Porter <mporter@kernel.crashing.org>
Cc: Li Yang <leoli@freescale.com>
Cc: Kumar Gala <galak@kernel.crashing.org>
Cc: Micha Nelissen <micha@neli.hopto.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>