usb: gadget: fastboot: detach usb just before rebooting

The patch fixes the following error when updating a BSH SMM S2 board:
3:72>Start Cmd:FB[-t 8000]: ucmd nand write ${loadaddr} nanddtb ${filesize}
3:72>Okay (0.023s)
3:72>Start Cmd:FB: reboot
3:72>Fail Bulk(R):LIBUSB_ERROR_IO(0s)

The "fastboot reboot" command detaches the USB when it still needs to be
used. So let's detach the USB just before the reset.

CC: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Fixes: 5f7e01e9 ("usb: gadget: fastboot: detach usb on reboot commands")
Suggested-by: Michael Trimarchi <michael@amarulasolutions.com>
Co-developed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>