usb: gadget: function: fix unused function warnings

Use __maybe_unused attribute to fix following compiler warnings:
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  CC      drivers/usb/gadget/function/f_mtp.o
drivers/usb/gadget/function/f_mtp.c:1219:12: warning: ‘mtp_bind_config’ defined but not used [-Wunused-function]
drivers/usb/gadget/function/f_mtp.c:1300:12: warning: ‘mtp_setup’ defined but not used [-Wunused-function]
  LD      drivers/usb/gadget/function/usb_f_mtp.o
  CC      drivers/usb/gadget/function/f_accessory.o
drivers/usb/gadget/function/f_accessory.c:1172:12: warning: ‘acc_bind_config’ defined but not used [-Wunused-function]
  LD      drivers/usb/gadget/function/usb_f_accessory.o
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Signed-off-by: Amit Pundir <amit.pundir@linaro.org>