usb: gadget: atmel: fix build warnings

This patch fixes the following build warnings:

drivers/usb/gadget/udc/atmel_usba_udc.c:2207:12: warning:
‘usba_udc_suspend’ defined but not used [-Wunused-function] static int
usba_udc_suspend(struct device *dev)
drivers/usb/gadget/udc/atmel_usba_udc.c:2236:12: warning:
‘usba_udc_resume’ defined but not used [-Wunused-function] static int
usba_udc_resume(struct device *dev)

Signed-off-by: Felipe Balbi <balbi@ti.com>