usb: gadget: at91_udc: Fix error path

In function at91udc_probe()
call put_device() when device_register() fails.

Signed-off-by: Rahul Ruikar <rahul.ruikar@gmail.com>
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>