usb: mtu3: disable vbus rise/fall interrupts of ltssm

The vbus rise & fall interrupts are used to enable and disable
U3 function of device automatically, this cause some issues when
class driver is initialized as deactivated, and will skip over
software-controlled connect by pullup(), but UDC wants to keep
disconnect until usb_gadget_activate() is called which calls
pullup() if needed. So we disable vbus rise & fall interrupts
and just use pullup() to enable & disable U3 function, and reset
mtu3 state when disconnect instead when vbus fall.

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