[iSCSI] Fix incorrect heuristic.

The only valid check for Expected Data Transfer Length is :
R and W  MUST  NOT both be 0 when the Expected Data Transfer Length and/or
Bidirectional Read Expected Data Transfer Length are not 0

Bug #10469
Change-Id: I5503a5ad524c4e237b143696fc8eda895bcb9172
Reviewed-on: https://code.wireshark.org/review/4514
Reviewed-by: Michael Mann <mmann78@netscape.net>