bnx2x: fix mem leak when command is unknown

In bnx2x_mcast_enqueue_cmd() we'll leak the memory allocated to
'new_cmd' if we hit the deafault case of the 'switch (cmd)'.
Add a 'kfree(new_cmd)' to that case to avoid the leak.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Acked-by: Dmitry Kravkov <dmitry@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>