IB/mad: Fix memory leak in switch handling in ib_mad_recv_done_handler()

If agent_send_response() returns an error, we shouldn't do anything
differently than if it succeeds; setting response to NULL just means
that the response buffer gets leaked.

Signed-off-by: Suresh Shelvapille <suri@baymicrosystems.com>
Signed-off-by: Hal Rosenstock <hal.rosenstock@gmail.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>