usb: xhci-mtk: fix ISOC error when interval is zero

If the interval equal zero, needn't round up to power of two
for the number of packets in each ESIT, so fix it.

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