net/mlx4_en: Use minimal rx and tx ring sizes on kdump kernel

When memory is limited (on kdump kernel), reduce size of rx and tx rings.
Also reduce the number of rx rings.

Signed-off-by: Alaa Hleihel <alaa@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>