scsi: zfcp: clarify function argument name for trace tag string

scsi: zfcp: clarify function argument name for trace tag string

v2.6.30 commit a211d4f5bc85 ("[SCSI] zfcp: replace current ERP logging with
a more convenient version") changed trace record distinguishing from a
numerical ID to a 7 character string called "trace tag". While starting to
use function arguments with different type and semantics, it did not change
the argument name accordingly.

v2.6.38 commit 953717b00440 ("[SCSI] zfcp: Redesign of the debug tracing
for recovery actions.") renamed variable names "id" into "tag" but only
within zfcp_dbf.*, not within zfcp_erp.c.

This was a bit confusing since the remainder of zfcp does use the term
"trace tag". Also "id" is quite generic and it's not obvious for what.
Just unify it consistently and use the "dbf" prefix to relate the arguments
to the code in zfcp_dbf.*.

Signed-off-by: Steffen Maier <maier@linux.ibm.com>
Reviewed-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>