usb: musb: musb_gadget: fix resource leakage in error path

In function musb_gadget_setup() call put_device()
when device_register() fails.

Signed-off-by: Rahul Ruikar <rahul.ruikar@gmail.com>
Acked-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>