usb: gadget: dummy_hcd: Fix error path

In function dummy_udc_probe()
call put_device() when device_register() fails.
also usb_get_hcd() put before device_register() after review comment
from Alan Stern.

Signed-off-by: Rahul Ruikar <rahul.ruikar@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>