fusb300_rdcxf() used only in this file.
Fix the following sparse warning:

drivers/usb/gadget/fusb300_udc.c:560:6: warning: symbol 'fusb300_rdcxf' was not declared. Should it be static?
Signed-off-by: NJingoo Han <jg1.han@samsung.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>