USB: serial: ti_usb_3410_5052: reduce drain delay to one char

The three-character drain delay was added by commit f1175daa ("USB:
ti_usb_3410_5052: kill custom closing_wait") when removing the custom
closing-wait implementation, which used a fixed 20 ms poll period and
drain delay.

This was likely a bit too conservative as a one-character timeout (e.g.
33 ms at 300 bps) should be enough to compensate for the lack of a
transmitter empty bit in the TUSB5052 line-status register.

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