net/mlx5e: Use IS_ERR() to check and simplify code

Use IS_ERR() and PTR_ERR() instead of PTR_ERR_OR_ZERO() to
simplify code, avoid redundant judgements.

Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Signed-off-by: Tang Bin <tangbin@cmss.chinamobile.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>