iscsi-target: fix invalid flags in text response

In case of multiple text responses iscsi-target
sets both 'F' and 'C' bit for the final text response
pdu, this issue happens because hdr->flags is not
zeroed out before ORing with 'F' bit.

This patch removes the | operator to fix this issue.

Signed-off-by: Varun Prakash <varun@chelsio.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>