cxgb4: Fix queue free path of ULD drivers

Setting sge_uld_rxq_info to NULL in free_queues_uld().
We are referencing sge_uld_rxq_info in cxgb_up(). This
will fix a panic when interface is brought up after a
ULDq creation failure.

Fixes: 94cdb8bb

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