cxgb4: add support for tx traffic scheduling classes

Add support to create tx traffic scheduling classes with specified
scheduling parameters.  Return an existing class if a match is found
with same scheduling parameters.

Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>