net/mlx5e: Cleanup MACsec uninitialization routine

The mlx5e_macsec_cleanup() routine has NULL pointer dereferencing if mlx5
device doesn't support MACsec (priv->macsec will be NULL).

While at it delete comment line, assignment and extra blank lines, so fix
everything in one patch.

Fixes: 1f53da67 ("net/mlx5e: Create advanced steering operation (ASO) object for MACsec")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>