usb: mtu3: flush cache for the first GPD when allocate GPD ring

When allocate the GPD ring, and tell its address to the controller, then
the driver starts or resumes the QMU, the controller will try to access
the first GPD, so need flush the first one to avoid wrong GPD status.

Reported-by: Xin Lin <Xin.Lin@mediatek.com>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>