net/mlx5e: Cleanup safe switch channels API by passing params

mlx5e_safe_switch_channels accepts new_chs as a parameter and opens new
channels in place, then copying them to priv->channels. It requires all
the callers to allocate space for this temporary storage of the new
channels.

This commit cleans up the API by replacing new_chs with new_params, a
meaningful subset of new_chs to be filled by the caller. The temporary
space for the new channels is allocated inside mlx5e_safe_switch_params
(a new name for mlx5e_safe_switch_channels). An extra copy of params is
made, but since it's control flow, it's not critical.

Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>