dfu: Detach the controller on error

In case anything errors out during the DFU registration, detach
the controller instead of bailing out right away. This way, the
controller can be reattached on next attempt.

Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Signed-off-by: Marek Vasut <marex@denx.de>