iscsi: automatically detect data segment digest

Modelled after ccf7ed00b62dcb63cdb3a9851baa767acbb94013 which detects
the header digest field, this patch adds auto-detection for the Data
Digest field which comes after the data segment.

Since the digest is now automatically detected, drop the three related
preferences.

Verified against scsi-osd-example-001.pcap (from SampleCaptures).

Change-Id: Icd89f1be58889c7ab70aca9dff7d3f99c8fe04d6
Reviewed-on: https://code.wireshark.org/review/2882
Reviewed-by: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: Anders Broman <a.broman58@gmail.com>