rapidio: add core mport removal support

Add common mport removal support functions into the RapidIO subsystem
core.

Changes to the existing mport registration process have been made to
avoid race conditions with active subsystem interfaces immediately after
mport device registration: part of initialization code from
rio_register_mport() have been moved into separate function
rio_mport_initialize() to allow to perform mport registration as the
final step of setup process.

Signed-off-by: Alexandre Bounine <alexandre.bounine@idt.com>
Cc: Matt Porter <mporter@kernel.crashing.org>
Cc: Aurelien Jacquiot <a-jacquiot@ti.com>
Cc: Andre van Herk <andre.van.herk@prodrive-technologies.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>