xhci: Fix null pointer dereference if xhci initialization fails

commit c207e7c50f31113c24a9f536fcab1e8a256985d7 upstream.

If xhci initialization fails before the roothub bandwidth
domains (xhci->rh_bw[i]) are allocated it will oops when
trying to access rh_bw members in xhci_mem_cleanup().

Reported-by: Manuel Reimer <manuel.reimer@gmx.de>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>