net/mlx5: DR, Fix NULL vs IS_ERR checking in dr_domain_init_resources

The mlx5_get_uars_page() function  returns error pointers.
Using IS_ERR() to check the return value to fix this.

Fixes: 4ec9e7b0

 ("net/mlx5: DR, Expose steering domain functionality")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>