cxgbit: Use type ISCSI_CXGBIT + cxgbit tpg_np attribute

Instead of having cxgbit use type ISCSI_HW_OFFLOAD + 'hw_offload'
tpg_np attribute, it should be using it's own driver specific
type + attribute

Cc: Varun Prakash <varun@chelsio.com>
Cc: Hariprasad Shenai <hariprasad@chelsio.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>