mlxsw: Fix mlxsw_i2c_write return value

The "err" variable is been checked, return always 0.

Signed-off-by: Elad Raz <eladr@mellanox.com>
Acked-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Vadim Pasternak <vadimp@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>