target: fix pr_out length in iscsi_parse_pr_out_transport_id

Old code in iscsi_parse_pr_out_transport_id() was obviously buggy
and effectively ignored the high byte.

Found by coverity.

Signed-off-by: Joern Engel <joern@logfs.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>