xhci: Fix NULL pointer dereference with xhci_irq() for shared_hcd

Commit ("f068090426ea xhci: Fix leaking USB3 shared_hcd
at xhci removal") sets xhci_shared_hcd to NULL without
stopping xhci host. This results into a race condition
where shared_hcd (super speed roothub) related interrupts
are being handled with xhci_irq happens when the
xhci_plat_remove is called and shared_hcd is set to NULL.
Fix this by setting the shared_hcd to NULL only after the
controller is halted and no interrupts are generated.

Change-Id: I2c7e460c171eacbf26d0bf7185d563ddb9efb504
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>