usb: gadget: printer: add missing error handling

If cdev_add() in printer_bind_config() fails, care is taken to
reverse the effects of initializations completed until the fail
happens. But if printer_req_alloc() fails, it is just one of the
two lists that is cleaned up while the effects of cdev_add()
and device_create() are not reverted.

This patch changes error handling so that at least as much cleanup is done
as when a failure happens before printer_req_alloc() invocations.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>