usb: dwc2: fix gadget disconnect

This fixes a disconnect issue detected with fastboot command, when using
dwc2 driver.
- On u-boot side:
uboot>$ fastboot 0
- On USB host PC side, few seconds after
PC>$ fastboot reboot # Get stuck, uboot target never reboots

By enabling DEBUG_ISR logs, the bus suspend interrupt is seen before the
PC command has been issued. When the USB bus suspend occurs, there's a HACK
that disables the fastboot (composite driver). Here is the call stack
upon USB bus suspend:
- dwc2_handle_usb_suspend_intr()
  - dev->driver->disconnect()
    - composite_disconnect()
      - reset_config()
        - f->disable()
          - fastboot_disable()
            - usb_ep_disable(f_fb->out_ep);
            - usb_ep_disable(f_fb->in_ep);
            .. other disable calls.

When the resume interrupt happens, everything has been disabled, then
nothing happens. fastboot command gets stuck on HOST side.

Remove original HACK, that disconnects the composite driver upon
USB bus suspend. Implement disconnect detection instead:
- check GINTSTS OTG interrupt
- read GOTGINT register
- check GOTGINT, SesEndDet bit (e.g. session end)
This is inspired by what is implemented currently in Linux dwc2 driver.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-by: Marek Vasut <marex@denx.de>