usb: mtu3: simplify getting .drvdata

We should get drvdata from struct device directly. Going via
platform_device is an unneeded step back and forth.

Acked-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>