usb: xhci-mtk: fix ptr_ret.cocci warnings

drivers/usb/host/xhci-mtk.c:256:1-3: WARNING: PTR_ERR_OR_ZERO can be used

 Use PTR_ERR_OR_ZERO rather than if(IS_ERR(...)) + PTR_ERR

Generated by: scripts/coccinelle/api/ptr_ret.cocci

Fixes: b6bb72cf ("usb: xhci-mtk: add optional mcu and dma bus clocks")
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Acked-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>