net/mlx5e: Fix FW error while setting IPsec policy block action

When trying to set IPsec policy block action the following error is
generated:

 mlx5_cmd_out_err:803:(pid 3426): SET_FLOW_TABLE_ENTRY(0x936) op_mod(0x0) failed,
	status bad parameter(0x3), syndrome (0x8708c3), err(-22)

This error means that drop action is not allowed when modify action is
set, so update the code to skip modify header for XFRM_POLICY_BLOCK action.

Fixes: 67212396 ("net/mlx5e: Skip IPsec encryption for TX path without matching policy")
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>