usb: xhci: convert to HCS_MAX_PORTS()

Use HCS_MAX_PORTS(p) instead of
((p & HCS_MAX_PORTS_MASK) >> HCS_MAX_PORTS_SHIFT)

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>