rapidio: use msleep in discovery wait

Use msleep() for code clarity as suggested by Andrew Morton in his
comments for the original patch: https://lkml.org/lkml/2012/10/3/546.

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>