net/mlx5e: Introduce mlx5e_channels API to get RQNs

Currently, struct mlx5e_channels is defined in en.h, along with a lot of
other stuff. In the following commit mlx5e_rx_res will need to get RQNs
(RQ hardware IDs), given a pointer to mlx5e_channels and the channel
index. In order to make it possible without including the whole en.h,
this commit introduces functions that will hide the implementation
details of mlx5e_channels.

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