net/mlx4: Set number of RX rings in a utility function

mlx4_en_add() is too long.
Moving set number of RX rings to a utiltity function to improve
readability and modulization of the code.

Signed-off-by: Ido Shamay <idos@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>