usb-serial: ftdi_sio: fix reference counting of ftdi_private

This patch (as1238) adds proper reference counting for ftdi_sio's
private data structure.  Without it, the driver will free the
structure while it is still in use if the user unplugs the serial
device before closing the device file.

The patch also replaces a slightly dangerous
cancel_delayed_work/flush_scheduled_work pair with
cancel_delayed_work_sync, which is always safer.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reported-by: Daniel Mack <daniel@caiaq.de>
Tested-by: Daniel Mack <daniel@caiaq.de>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>