net/mlx5e: Introduce abstraction of RSS context

Bring all fields that define and maintain RSS behavior together
into a new structure.
Align all usages with this new structure. Keep it hidden within
rx_res.c.
This helps supporting multiple RSS contexts in downstream patch.

Use dynamic allocations for the RSS context.

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