USB: serial: spcp8x5: fix modem-status handling

Make sure to detect short control transfers and return zero on success
when retrieving the modem status.

This fixes the TIOCMGET implementation which since e1ed212d8593 ("USB:
spcp8x5: add proper modem-status support") has returned TIOCM_LE on
successful retrieval, and avoids leaking bits from the stack on short
transfers.

This also fixes the carrier-detect implementation which since the above
mentioned commit unconditionally has returned true.

Fixes: e1ed212d8593 ("USB: spcp8x5: add proper modem-status support")
Cc: stable <stable@vger.kernel.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/spcp8x5.c

@@ -232,11 +232,17 @@ static int spcp8x5_get_msr(struct usb_serial_port *port, u8 *status)
 	ret = usb_control_msg(dev, usb_rcvctrlpipe(dev, 0),
 			      GET_UART_STATUS, GET_UART_STATUS_TYPE,
 			      0, GET_UART_STATUS_MSR, buf, 1, 100);
-	if (ret < 0)
+	if (ret < 1) {
 		dev_err(&port->dev, "failed to get modem status: %d\n", ret);
+		if (ret >= 0)
+			ret = -EIO;
+		goto out;
+	}
 
 	dev_dbg(&port->dev, "0xc0:0x22:0:6  %d - 0x02%x\n", ret, *buf);
 	*status = *buf;
+	ret = 0;
+out:
 	kfree(buf);
 
 	return ret;