usb: xhci: Allow usb_add/remove_hcd() to be called repeatedly

Don't set xhci->shared_hcd to NULL in xhci_stop() as we have
still not de-allocated it. It was resulting in a NULL pointer
de-reference if usb_add/remove_hcd() is called repeatedly.

We want repeated add/remove to work for the OTG use case.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>