cxgb4: remove dead code when allocating filter

Error code is already returned earlier if filter exists
at specified location. So, remove dead code trying to
free existing filter.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>