qla2xxx: Enable Exchange offload support.

This patch enables Exchange offload support in Qlogic ISP.
To enable exchange offload with Qlogic ISP24XX/25XX/26XX,
set module parameter ql2xexchoffld to any non-zero number.
This will alow ISP firmware to store exchange data structures
used by firmware to host memory provided by driver. ISP firmware
can supports upto 32k total active exchanges.

Signed-off-by: Himanshu Madhani <himanshu.madhani@qlogic.com>
Signed-off-by: Giridhar Malavali <giridhar.malavali@qlogic.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>