USB: serial: ti_usb_3410_5052: drop drain delay for 3410

Unlike the TUSB5052, the TUSB3410 has an LSR TEMT bit to tell if both
the transmitter data and shift registers are empty.

Make sure to check also the shift register on TUSB3410 when waiting for
the transmit buffer to drain during close and drop the time-based
one-char delay which is otherwise needed (e.g. 90 ms at 110 bps).

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>