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

drivers/usb/gadget/fusb300_udc.c

@@ -557,7 +557,7 @@ static void fusb300_set_cxdone(struct fusb300 *fusb300)
 }
 
 /* read data from cx fifo */
-void fusb300_rdcxf(struct fusb300 *fusb300,
+static void fusb300_rdcxf(struct fusb300 *fusb300,
 		   u8 *buffer, u32 length)
 {
 	int i = 0;