USB: serial: ark3116: fix register-accessor error handling

commit 9fef37d7 upstream.

The current implementation failed to detect short transfers, something
which could lead to bits of the uninitialised heap transfer buffer
leaking to user space.

Fixes: 149fc791 ("USB: ark3116: Setup some basic infrastructure for
new ark3116 driver.")
Fixes: f4c1e8d5 ("USB: ark3116: Make existing functions 16450-aware
and add close and release functions.")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/usb/serial/ark3116.c

@@ -107,10 +107,17 @@ static int ark3116_read_reg(struct usb_serial *serial,
 				 usb_rcvctrlpipe(serial->dev, 0),
 				 0xfe, 0xc0, 0, reg,
 				 buf, 1, ARK_TIMEOUT);
-	if (result < 0)
+	if (result < 1) {
+		dev_err(&serial->interface->dev,
+				"failed to read register %u: %d\n",
+				reg, result);
+		if (result >= 0)
+			result = -EIO;
+
 		return result;
-	else
-		return buf[0];
+	}
+
+	return buf[0];
 }
 
 static inline int calc_divisor(int bps)