[SCSI] zfcp: Simplify latency lock handling

The lock only needs to protect the softirq context called from qdio
against the userspace context called from sysfs. spin_lock and
spin_lock_bh is enough.

Acked-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>