MdeModulePkg/Xhci: Fill the 'interval' field for ISO endpoint context
    
    https://bugzilla.tianocore.org/show_bug.cgi?id=510
    
    The commit fills the 'Interval' field of the Endpoint Context data for
    isochronous endpoints. It will resolve the error when a Configure
    Endpoint Command is sent to an isochronous endpoint.
    
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Hao Wu <hao.a.wu@intel.com>
    Reviewed-by: Star Zeng <star.zeng@intel.com>