EHCI USB: Don't use generic shutdown function

The OMAP EHCI glue code has a layer of driver state struct between
the platform_device and usb_hcd. So it can't use the generic
usb_hcd_platform_shutdown.

This fixes a panic at reboot time.

Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>