usb: mtu3: fix operation failure when test TEST_J/K

There is an error dialog popped up in PC when test TEST_J/K
by EHSETT tool, due to not waiting for the completion of
control transfer. Here fix it by entering test mode after
Status Stage finish.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>