usb: xhci: Honor endpoint's interval

USB endpoint reports the period between consecutive requests to send
or receive data as bInverval in its endpoint descriptor. So far this
is ignored by xHCI driver and the 'Interval' field in xHC's endpoint
context is always programmed to zero which means 1ms for low speed
or full speed , or 125us for high speed or super speed. We should
honor the interval by getting it from endpoint descriptor.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>