xhci: Remove references to HC_STATE_HALT.

The xHCI driver doesn't ever test hcd->state for HC_STATE_HALT.  The USB
core recently stopped using it internally, so there's no point in setting
it in the driver.  We still need to set HC_STATE_RUNNING in order to make
it past the USB core's hcd->state check in register_roothub().

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