xhci: rename EP_HALT_PENDING to EP_STOP_CMD_PENDING

We don't want to confuse halted and stalled endpoint states with
a flag indicating we are waiting for a stop endpoint command to
finish or timeout

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