net/mlx5_core: Remove redundant "err" variable usage

Cosmetic change.
Do not use the an err variable just to assign and return it.

Signed-off-by: Achiad Shochat <achiad@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>