USB: serial: cp210x: fix tiocmget error handling

The current tiocmget implementation would fail to report errors up the
stack and instead leaked a few bits from the stack as a mask of
modem-status flags.

Fixes: 39a66b8d22a3 ("[PATCH] USB: CP2101 Add support for flow control")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/cp210x.c

@@ -1077,7 +1077,9 @@ static int cp210x_tiocmget(struct tty_struct *tty)
 	u8 control;
 	int result;
 
-	cp210x_read_u8_reg(port, CP210X_GET_MDMSTS, &control);
+	result = cp210x_read_u8_reg(port, CP210X_GET_MDMSTS, &control);
+	if (result)
+		return result;
 
 	result = ((control & CONTROL_DTR) ? TIOCM_DTR : 0)
 		|((control & CONTROL_RTS) ? TIOCM_RTS : 0)