qed: Add a missing error code

We should be returning -ENOMEM if qed_mcp_cmd_add_elem() fails.  The
current code returns success.

Fixes: 4ed1eea8 ("qed: Revise MFW command locking")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Tomer Tayar <Tomer.Tayar@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>