net/mlx5e: Introduce mlx5e_channels

Have a dedicated "channels" handler that will serve as channels
(RQs/SQs/etc..) holder to help with separating channels/parameters
operations, for the downstream fail-safe configuration flow, where we will
create a new instance of mlx5e_channels with the new requested parameters
and switch to the new channels on the fly.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>