USB: serial: mos7840: fix control-message error handling

Make sure to detect short transfers when reading a device register.

The modem-status handling had sufficient error checks in place, but move
handling of short transfers into the register accessor function itself
for consistency.
Reviewed-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NJohan Hovold <johan@kernel.org>

drivers/usb/serial/mos7840.c

@@ -284,9 +284,15 @@ static int mos7840_get_reg_sync(struct usb_serial_port *port, __u16 reg,
 	ret = usb_control_msg(dev, usb_rcvctrlpipe(dev, 0), MCS_RDREQ,
 			      MCS_RD_RTYPE, 0, reg, buf, VENDOR_READ_LENGTH,
 			      MOS_WDR_TIMEOUT);
+	if (ret < VENDOR_READ_LENGTH) {
+		if (ret >= 0)
+			ret = -EIO;
+		goto out;
+	}
+
 	*val = buf[0];
 	dev_dbg(&port->dev, "%s offset is %x, return val %x\n", __func__, reg, *val);
-
+out:
 	kfree(buf);
 	return ret;
 }
@@ -352,8 +358,13 @@ static int mos7840_get_uart_reg(struct usb_serial_port *port, __u16 reg,
 	ret = usb_control_msg(dev, usb_rcvctrlpipe(dev, 0), MCS_RDREQ,
 			      MCS_RD_RTYPE, Wval, reg, buf, VENDOR_READ_LENGTH,
 			      MOS_WDR_TIMEOUT);
+	if (ret < VENDOR_READ_LENGTH) {
+		if (ret >= 0)
+			ret = -EIO;
+		goto out;
+	}
 	*val = buf[0];
-
+out:
 	kfree(buf);
 	return ret;
 }
@@ -1479,10 +1490,10 @@ static int mos7840_tiocmget(struct tty_struct *tty)
 		return -ENODEV;
 
 	status = mos7840_get_uart_reg(port, MODEM_STATUS_REGISTER, &msr);
-	if (status != 1)
+	if (status < 0)
 		return -EIO;
 	status = mos7840_get_uart_reg(port, MODEM_CONTROL_REGISTER, &mcr);
-	if (status != 1)
+	if (status < 0)
 		return -EIO;
 	result = ((mcr & MCR_DTR) ? TIOCM_DTR : 0)
 	    | ((mcr & MCR_RTS) ? TIOCM_RTS : 0)