usbus/dfu: cannot detach device to reboot into bootloader

[dylad]

Description

When dfu is in used, one can use dfu-util -e from a shell when the application is running to perform a reset into bootloader. (if USEMODULE=usbus_dfu) is present.
However, in current master, the operation fails.
It seems this regression was introduced by #17090.

Steps to reproduce the issue

Flash riotboot_dfu bootloader app:
make BOARD=same54-xpro -C bootloaders/riotboot_dfu flash
Flash an application on riotboot/slot0:
FEATURES_REQUIRED=riotboot PROGRAMMER=dfu-util USEMODULE=usbus_dfu make -j8 BOARD=same54-xpro -C tests/sys/shell riotboot/flash-slot0
Try to flash another application:
DFU_USB_ID=1209:7d00 FEATURES_REQUIRED=riotboot PROGRAMMER=dfu-util USEMODULE=usbus_dfu make -j8 BOARD=same54-xpro -C tests/leds riotboot/flash-slot1

Expected results

The last application should be flashed properly and running afterwards.

Actual results

Flashing fails because device doesn't reboot into bootloader.

[dylad]

I think I've identified the issue.
I'll prepare a patch in the following days.