fabrics: check for discovery controller instead of subsystem NQN

fabrics: check for discovery controller instead of subsystem NQN

Unique discovery subsystems have a unique subsystem NQN, to we cannot
check for the standard discovery NQN when checking for persistent
discovery controllers, but rather have to check if the subsystem is a
discovery subsystem.

Signed-off-by: Hannes Reinecke <hare@suse.de>