usb: gadget: f_midi: unlock on error

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

Fixes: 9acdf4df2fc4 ('usb: gadget: f_midi: added spinlock on transmit function')
Acked-by: Michal Nazarewicz <[e-mail]>
Acked-by: Felipe F. Tonello <[e-mail]>
Signed-off-by: Dan Carpenter <[e-mail]>
Signed-off-by: Felipe Balbi <[e-mail]>

drivers/usb/gadget/function/f_midi.c

···
 
 	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);