usbip: vhci-hcd: Rework vhci_hcd_init

A vhci struct is added as the platform-specific data to the vhci
platform device, in order to get the vhci by its platform device.
This is done in vhci_hcd_init().

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>