USB: ftdi_sio: fix chars_in_buffer overhead

USB: ftdi_sio: fix chars_in_buffer overhead

Use the new generic usb-serial wait_until_sent implementation to wait
for hardware buffers to drain.

This removes the need to check the hardware buffers in chars_in_buffer
and thus removes the overhead introduced by commit 6f602912 ("usb:
serial: ftdi_sio: Add missing chars_in_buffer function") without
breaking tty_wait_until_sent (used by, for example, tcdrain, tcsendbreak
and close).

Reported-by: Stas Sergeev <stsp@list.ru>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>