usb: mtu3: fix ip sleep auto-exit issue when enable DRD mode

Ip sleep will auto exit if vbus comparison circuit of u2 phy is
disabled when system tries to enter suspend mode, so get vbus-valid
status from mac but not from u2 phy when enable DRD mode to fix
the issue.

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