usb: gadget: fix two sparse warnings

drivers/usb/gadget/u_serial.c:1291:5: sparse: symbol \
	'userial_init' was not declared. Should it be static?
drivers/usb/gadget/zero.c:66:25: sparse: symbol \
	'gzero_options' was not declared. Should it be static?

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