net/mlx4_en: Cleanups suggested by clang static checker

clang flagged the following. All are actually cosmetic cleanups, not really bugs:

drivers/net/ethernet/mellanox/mlx4/en_main.c:233:3: warning: Value stored to 'err' is never read
                err = -ENOMEM;
                ^     ~~~~~~~
drivers/net/ethernet/mellanox/mlx4/en_main.c:293:3: warning: Value stored to 'err' is never read
                err = -ENOMEM;

drivers/net/ethernet/mellanox/mlx4/en_netdev.c:648:16: warning: Assigned value is garbage or undefined
        entry->reg_id = reg_id;
                      ^ ~~~~~~
drivers/net/ethernet/mellanox/mlx4/en_netdev.c:659:2: warning: Function call argument is an uninitialized value
        mlx4_en_uc_steer_release(priv, priv->dev->dev_addr, *qpn, reg_id);
(NOTE: reg_id is only used in the device-managed flow steering path, in which is it always initialized.
 This is not a bug. Cleanup here is therefore cosmetic only).

drivers/net/ethernet/mellanox/mlx4/en_rx.c:122:3: warning: Value stored to 'frag_info' is never read
                frag_info = &priv->frag_info[i];
                ^           ~~~~~~~~~~~~~~~~~~~

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>

Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>