[SCSI] zfcp: Replace config semaphore with mutex

The config semaphore is only used as a mutex, so replace it with a
simple mutex.

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