[SCSI] zfcp: channel cannot be detached due to refcount imbalance

Due to an imbalance of the nameserver refcounting, channel devices
couldn't be take offline anymore after the channel was activated.

Signed-off-by: Swen Schillig <swen@vnet.ibm.com>
Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>