USB: serial: ftdi_sio: add support for TIOCSERGETLSR

Willem-Jan noticed that the ftdi_sio driver did not support the
TIOCSERGETLSR ioctl, and some userspace programs rely on it.  This patch
adds the support.

Reported-by: Willem-Jan de Hoog <wdehoog@exalondelft.nl>
Tested-by: Willem-Jan de Hoog <wdehoog@exalondelft.nl>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>