cxgb4: Fix uld_send() for ctrl pkts

Without any uld being loaded, uld_txq_info[] will be NULL. uld_send()
is also used for sending control work requests(for eg: setting filter)
that dont require any ulds to be loaded. Hence move uld_txq_info[]
assignment after ctrl_xmit().

Also added a NULL check for uld_txq_info[].

Fixes: 94cdb8bb

 (cxgb4: Add support for dynamic allocation
       of resources for ULD).
Signed-off-by: Arjun V <arjun@chelsio.com>
Signed-off-by: Casey Leedom <leedom@chelsio.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>