iw_cxgb4: Use dsgl by default

Enable the use of dsgl by default and determine whether dsgl is
supported from lld info.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Bharat Potnuri <bharat@chelsio.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>