[SCSI] zfcp: Simplify zfcp_dbf_tag and related functions in debug trace.

Simplify usage of zfcp_dbf_tag() and calling functions.

Signed-off-by: Martin Peschke <mp3@de.ibm.com>
Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>