inert_drv: reset the mode before apply a new mode

driver_select() OR's together the modes from successive calls:

    1> {ok,R} = inert:start().
    {ok,<0.35.0>}
    2> {ok,S} = gen_tcp:listen(7070, [binary, {active,false}]).
    {ok,#Port<0.955>}
    3> inert:fdset(R, 7, [{mode, read_write}]).
    ok
    4> flush().
    ok
    5> inert:fdset(R, 7, [{mode, write}]).
    ok
    % In another term: nc localhost 7070
    6> flush().
    Shell got {inert_read,<0.35.0>,7}