usb: gadget: f_midi: unlock on error

We added some new locking here, but missed an error path where we need
to unlock.

Fixes: 9acdf4df ('usb: gadget: f_midi: added spinlock on transmit function')
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Acked-by: Felipe F. Tonello <eu@felipetonello.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/function/f_midi.c

@@ -611,8 +611,10 @@ static void f_midi_transmit(struct f_midi *midi)
 
 	do {
 		ret = f_midi_do_transmit(midi, ep);
-		if (ret < 0)
+		if (ret < 0) {
+			spin_unlock_irqrestore(&midi->transmit_lock, flags);
 			goto drop_out;
+		}
 	} while (ret);
 
 	spin_unlock_irqrestore(&midi->transmit_lock, flags);