usb: dwc3: no need to clear INIT{U1,U2}ENA bits

as per databook, these bits are cleared by hardware on each USB reset,
so no need to clear it explicitly by software in reset ISR.

Signed-off-by: Pratyush Anand <pratyush.anand@st.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>