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>