xhci: Clean up work to xhci_add_endpoint().

This patch removes unused variable "out_ctx" and avoid multiple calls
to function xhci_get_endpoint_flag().

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