net/mlx5e: Prevent casting overflow

On 64 bits architectures unsigned long is longer than u32,
casting to unsigned long will result in overflow.
We need to first allocate an unsigned long variable, then assign the
wanted value.

Fixes: 665bc539 ('net/mlx5e: Use new ethtool get/set link ksettings API')
Signed-off-by: Gal Pressman <galp@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>