usb: gadget: fastboot: detach usb on reboot commands

When host issues "fastboot reboot fastboot", it's expected that the
board drops the USB connection before resetting.

On some boards, such as Khadas VIM3L and SEI610, this is not the case.

We observe the following error:
$ fastboot reboot fastboot
Rebooting into fastboot                            OKAY [  0.004s]
fastboot: error: Failed to boot into userspace fastboot; one or more components might be unbootable.

This does not happen when we use the RST button on the board.
It can be reproduced in linux with:
  # echo b > /proc/sysrq-trigger

In this case, we hit a undefined hardware behavior, where D+ and D-
are in an unknown state. Therefore the host can't detect usb
disconnection.

Make sure we always call usb_gadget_release() when a "fastboot reboot"
command is issued.

Note: usb_gadget_release() should be called before g_dnl_unregister()
because g_dnl_unregister() triggers a complete() call on each
endpoint (thus calling do_reset()).

Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>