usb: phy: mv-u3d: Staticize mv_u3d_phy_shutdown()

mv_u3d_phy_shutdown() is used only in this file.
Fix the following sparse warning:

drivers/usb/phy/phy-mv-u3d-usb.c:85:6: warning: symbol 'mv_u3d_phy_shutdown' was not declared. Should it be static?
Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/phy/phy-mv-u3d-usb.c

@@ -82,7 +82,7 @@ static void mv_u3d_phy_write(void __iomem *base, u32 reg, u32 value)
 	writel_relaxed(value, data);
 }
 
-void mv_u3d_phy_shutdown(struct usb_phy *phy)
+static void mv_u3d_phy_shutdown(struct usb_phy *phy)
 {
 	struct mv_u3d_phy *mv_u3d_phy;
 	void __iomem *base;