cxgb4: add definitions for iSCSI target ULD

add structure, macro and constant definitions
for iSCSI Tx and Rx.

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