usb: gadget: udc: start_udc() can be static

Fixes the following Sparse warnings:

>> drivers/usb/gadget/udc/snps_udc_plat.c:31:6: sparse: symbol 'start_udc' was not declared. Should it be static?
>> drivers/usb/gadget/udc/snps_udc_plat.c:41:6: sparse: symbol 'stop_udc' was not declared. Should it be static?
>> drivers/usb/gadget/udc/snps_udc_plat.c:79:6: sparse: symbol 'udc_drd_work' was not declared. Should it be static?

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>