net/mlx5: Fix mlx5_get_uars_page to return error code

Change mlx5_get_uars_page to return ERR_PTR in case of
allocation failure. Change all callers accordingly to
check the IS_ERR(ptr) instead of NULL.

Fixes: 59211bd3 ("net/mlx5: Split the load/unload flow into hardware and software flows")
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>