drivers/rapidio/rio-scan.c: use discovered bit to test if enumeration is complete

The discovered bit in PGCCSR register indicates if the device has been
discovered by system host.  In Rapidio systems, some agent devices can also
be master devices.  They can issue requests into the system.

Signed-off-by: Liu Gang <Gang.Liu@freescale.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>