usb: xhci: Avoid accessing dangling virt_dev in xhci_stop_device

In xhci_stop_device() the virt_dev pointer is assigned from the
device slot prior to acquiring xhci->lock and disabling IRQs.
This is an opportunity for a TRB_DISABLE_SLOT command to have
completed (e.g. in case the device was disconnected, or the port
was disabled) which would have called xhci_free_virt_device()
and freed the slot's pointer in IRQ context. Then after the
lock is acquired virt_dev may now be pointing to freed memory
and results in a panic.

Avoid this by moving the assignment

	virt_dev = xhci->devs[slot_id];

and NULL pointer check to after the lock is taken to ensure that
we don't race against xhci IRQ handler.

Change-Id: I4ac23fbfd3def0ad93967316b62c0c0cd19ca33f
Signed-off-by: Jack Pham <jackp@codeaurora.org>