tty: Fix double grabbing of a spinlock

The HSO changes for kref introduced a recursive spinlock take. All
functions which call put_rxbuf_data already have serial->serial_lock
grabbed.

[Comment to code added-AC]
Signed-off-by: NDenis Joseph Barrow <D.Barrow@option.com>
Signed-off-by: NAlan Cox <alan@redhat.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/net/usb/hso.c

@@ -2044,9 +2044,8 @@ static int put_rxbuf_data(struct urb *urb, struct hso_serial *serial)
 		return -2;
 	}
 
-	spin_lock(&serial->serial_lock);
+	/* All callers to put_rxbuf_data hold serial_lock */
 	tty = tty_kref_get(serial->tty);
-	spin_unlock(&serial->serial_lock);
 
 	/* Push data to tty */
 	if (tty) {