[SCSI] zfcp: Remove scsi_cmnd->serial_number from debug traces

With the change that drivers have to explicitly request the serial
number for SCSI commands, this field should not be part of the zfcp
traces. It is not worth the effort to request the serial number only
for tracing purposes, so simply remove this field from the debug
traces.

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>