USB: ti_usb_3410_5052: kill custom closing_wait

Kill custom closing_wait implementation and let the tty-layer handle it
instead.

Note that the port drain-delay is set to three characters to keep the
20ms delay after wait_until_sent at low baudrates (1200 baud) during close.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>