USB: ci13xxx_udc: make suspend and resume in gadget driver optional

Some gadget drivers don't implement suspend and/or resume functions.
Instead of changing the gadget drivers, make suspend and resume in
ci13xxx_udc (following other udc drivers) optional.

Tested-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>