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>