USB: serial: ftdi_sio: fix modem-status error handling

Make sure to detect short responses when fetching the modem status in
order to avoid parsing uninitialised buffer data and having bits of it
leak to user space.

Note that we still allow for short 1-byte responses.

Fixes: 1da177e4

 ("Linux-2.6.12-rc2")
Cc: stable <stable@vger.kernel.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>