usb: gadget: ci13xxx: convert to new style

Finally, convert to the new style framework, using udc_start/udc_stop
methods. Since there is no need in the global _udc pointer, remove it.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>