net/mlx5e: Move generic functions to new file

These functions were identified as ones that could be made generic and
used by multiple drivers.  Most of the contents of en_rx_am.c are moved
to net_dim.c.

Signed-off-by: Andy Gospodarek <gospo@broadcom.com>
Acked-by: Tal Gilboa <talgi@mellanox.com>
Acked-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>