iscsi: minor retval fix

The return value of new dissectors is the number of bytes that were
successfully dissected, not a boolean. Fix that and get rid of an
unnecessary iSCSIPdusDissected variable.

Change-Id: Ie31df393a1eb44f185d320a4c2d35f5e8b7d7bd9
Reviewed-on: https://code.wireshark.org/review/2889
Reviewed-by: Evan Huus <eapache@gmail.com>