usb: gadget: fusb300_udc: Staticize fusb300_rdcxf()

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: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>