usbip: vhci-hcd: Move VHCI platform device into vhci struct

Every VHCI is a platform device, so move the platform_device struct
into the VHCI struct.

Signed-off-by: Yuyang Du <yuyang.du@intel.com>
Acked-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>