usb: gadget: serial: only call into CONSOLE_FULL code if it's there
Fixes:
drivers/built-in.o: In function `gserial_connect':
drivers/usb/gadget/u_serial.c:539: undefined reference to `console_set_active'
Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>