xhci: Return canceled URBs immediately when host is halted.

When the xHCI host controller is halted, it won't respond to commands
placed on the command ring.  So if an URB is cancelled after the first
roothub is deallocated, it will try to place a stop endpoint command on
the command ring, which will fail.  The command watchdog timer will fire
after five seconds, and the host controller will be marked as dying, and
all URBs will be completed.

Add a flag to the xHCI's internal state variable for when the host
controller is halted.  Immediately return the canceled URB if the host
controller is halted.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>