cxgb4: add support for per queue tx scheduling

Add support to bind/unbind specified tx queues to/from scheduling
classes.  If a queue is already bound to a scheduling class, it is
unbound first and then bound to a new specified class.

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>