USB: ftdi_sio: re-implement read processing

- Re-structure read processing.
 - Kill obsolete work queue and always push to tty in completion handler.
 - Use tty_insert_flip_string instead of per character push when
   possible.
 - Fix stalled-read regression in 2.6.31 by using urb status to
   determine when port is closed rather than port count.
 - Fix race with open/close by checking ASYNCB_INITIALIZED in
   unthrottle.
 - Kill private rx_flag and lock and use throttle flags in
   usb_serial_port instead.
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>