xhci: Don't add a virt_dev to the devs array before it's fully allocated

Avoid null pointer dereference if some function is walking through the
devs array accessing members of a new virt_dev that is mid allocation.

Add the virt_dev to xhci->devs[i] _after_ the virt_device and all its
members are properly allocated.

issue found by KASAN: null-ptr-deref in xhci_find_slot_id_by_port

"Quick analysis suggests that xhci_alloc_virt_device() is not mutex
protected. If so, there is a time frame where xhci->devs[slot_id] is set
but not fully initialized. Specifically, xhci->devs[i]->udev can be NULL."

Cc: stable <stable@vger.kernel.org>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>