USB: serial: ftdi_sio: fix GPIO not working in autosuspend

There is a bug in the current GPIO code for ftdi_sio: it failed to take USB
autosuspend into account. If the device is in autosuspend, calls to
usb_control_msg() fail with -EHOSTUNREACH. Because the standard value for
autosuspend timeout is usually 2-5 seconds, this made it almost impossible
to use the GPIOs on machines that have USB autosuspend enabled. This patch
fixes the issue by acquiring a PM lock on the device for the duration of
the USB transfers. Tested on an FT231X device.
Signed-off-by: Karoly Pados <pados@pados.hu>
[ johan: simplify code somewhat ]
Fixes: ba93cc7d ("USB: serial: ftdi_sio: implement GPIO support for FT-X devices")
Cc: stable <stable@vger.kernel.org>	# 4.20
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/ftdi_sio.c

@@ -1783,6 +1783,10 @@ static int ftdi_set_bitmode(struct usb_serial_port *port, u8 mode)
 	int result;
 	u16 val;
 
+	result = usb_autopm_get_interface(serial->interface);
+	if (result)
+		return result;
+
 	val = (mode << 8) | (priv->gpio_output << 4) | priv->gpio_value;
 	result = usb_control_msg(serial->dev,
 				 usb_sndctrlpipe(serial->dev, 0),
@@ -1795,6 +1799,8 @@ static int ftdi_set_bitmode(struct usb_serial_port *port, u8 mode)
 			val, result);
 	}
 
+	usb_autopm_put_interface(serial->interface);
+
 	return result;
 }
 
@@ -1846,9 +1852,15 @@ static int ftdi_read_cbus_pins(struct usb_serial_port *port)
 	unsigned char *buf;
 	int result;
 
+	result = usb_autopm_get_interface(serial->interface);
+	if (result)
+		return result;
+
 	buf = kmalloc(1, GFP_KERNEL);
-	if (!buf)
+	if (!buf) {
+		usb_autopm_put_interface(serial->interface);
 		return -ENOMEM;
+	}
 
 	result = usb_control_msg(serial->dev,
 				 usb_rcvctrlpipe(serial->dev, 0),
@@ -1863,6 +1875,7 @@ static int ftdi_read_cbus_pins(struct usb_serial_port *port)
 	}
 
 	kfree(buf);
+	usb_autopm_put_interface(serial->interface);
 
 	return result;
 }