net/mlx5e: RX, Re-work initializaiton of RX function pointers

Instead of exposing the RQ datapath handlers (from en_rx.c) so that
they are set in the control path (in en_main.c), wrap this logic
in a single function in en_rx.c and expose it alone.

Every profile will now have a pointer to the new mlx5e_rx_handlers
structure, instead of directly pointing to the previously-exposed
RQ handlers.

This significantly improves locality and modularity of the driver,
and allows many functions in en_rx.c to become static.

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