usb: renesas_usbhs: make usbhs_write32() static

The usbhs_write32 function is not used outside of the rcar3.c
file, so fix the following sparse warning by making it static:

drivers/usb/renesas_usbhs/rcar3.c:26:6: warning: symbol 'usbhs_write32' was not declared. Should it be static?
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/renesas_usbhs/rcar3.c

@@ -23,7 +23,7 @@
 #define UGCTRL2_RESERVED_3	0x00000001	/* bit[3:0] should be B'0001 */
 #define UGCTRL2_USB0SEL_OTG	0x00000030
 
-void usbhs_write32(struct usbhs_priv *priv, u32 reg, u32 data)
+static void usbhs_write32(struct usbhs_priv *priv, u32 reg, u32 data)
 {
 	iowrite32(data, priv->base + reg);
 }