USB: serial: fix runtime PM after driver unbind

BugLink: https://bugs.launchpad.net/bugs/1848780

commit d51bdb93 upstream.

Since commit c2b71462 ("USB: core: Fix bug caused by duplicate
interface PM usage counter") USB drivers must always balance their
runtime PM gets and puts, including when the driver has already been
unbound from the interface.

Leaving the interface with a positive PM usage counter would prevent a
later bound driver from suspending the device.

Fixes: c2b71462 ("USB: core: Fix bug caused by duplicate interface PM usage counter")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20191001084908.2003-4-johan@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

drivers/usb/serial/usb-serial.c

@@ -314,10 +314,7 @@ static void serial_cleanup(struct tty_struct *tty)
 	serial = port->serial;
 	owner = serial->type->driver.owner;
 
-	mutex_lock(&serial->disc_mutex);
-	if (!serial->disconnected)
 	usb_autopm_put_interface(serial->interface);
-	mutex_unlock(&serial->disc_mutex);
 
 	usb_serial_put(serial);
 	module_put(owner);