[SCSI] zfcp: simplify zfcp_dbf_timestamp()

Change zfcp_dbf_timestamp() so that it just calculates timespec from
timestamp. First step to be able to rip this code out of zfcp.
Besides, this change makes it easier to rip out old-style debug view
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>