xhci: Don't submit commands when the host is dead.

When the xHCI host controller dies, the USB core may attempt to reset the
devices to their default configuration before disconnecting them.  This
causes calls into the xHCI bandwidth allocation functions.  Don't allow
those functions to submit commands or work on xHCI structures if the host
controller is marked as dying.

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