xhci: Fix null pointer dereference in remove if xHC has only one roothub

The remove path in xhci platform driver tries to remove and put both main
and shared hcds even if only a main hcd exists (one roothub)

This causes a null pointer dereference in reboot for those controllers.

Check that the shared_hcd exists before trying to remove it.

Fixes: e0fe9869

 ("usb: host: xhci-plat: prepare operation w/o shared hcd")
Reported-by: Alexey Sheplyakov <asheplyakov@basealt.ru>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20220825150840.132216-2-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>