net/mlx4_en: Support for configurable RSS hash function

The ConnectX HW is capable of using one of the following hash functions:
Toeplitz and an XOR hash function. This patch extends the implementation
of the mlx4_en driver set/get_rxfh callbacks to support getting and
setting the RSS hash function used by the device.

Signed-off-by: Eyal Perry <eyalpe@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>