USB: g_serial: don't set low_latency flag

No longer set low_latency flag as it causes this warning backtrace:

  WARNING: at kernel/mutex.c:207 __mutex_lock_slowpath+0x6c/0x288()

Fix associated locking and wakeups.
Signed-off-by: Jon Povey <jon.povey@racelogic.co.uk>
Cc: Maulik Mankad <x0082077@ti.com>
Cc: stable <stable@kernel.org>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/gadget/u_serial.c

@@ -536,17 +536,11 @@ static void gs_rx_push(unsigned long _port)
 		list_move(&req->list, &port->read_pool);
 	}
 
-	/* Push from tty to ldisc; this is immediate with low_latency, and
-	 * may trigger callbacks to this driver ... so drop the spinlock.
+	/* Push from tty to ldisc; without low_latency set this is handled by
+	 * a workqueue, so we won't get callbacks and can hold port_lock
 	 */
 	if (tty && do_push) {
-		spin_unlock_irq(&port->port_lock);
 		tty_flip_buffer_push(tty);
-		wake_up_interruptible(&tty->read_wait);
-		spin_lock_irq(&port->port_lock);
-
-		/* tty may have been closed */
-		tty = port->port_tty;
 	}
 
 
@@ -784,11 +778,6 @@ static int gs_open(struct tty_struct *tty, struct file *file)
 	port->open_count = 1;
 	port->openclose = false;
 
-	/* low_latency means ldiscs work in tasklet context, without
-	 * needing a workqueue schedule ... easier to keep up.
-	 */
-	tty->low_latency = 1;
-
 	/* if connected, start the I/O stream */
 	if (port->port_usb) {
 		struct gserial	*gser = port->port_usb;