usb: dfu: fix detach happening too fast

In some relatively rare conditions, the DFU detach/attach happens to
fast, which cause the host to not notice it. It seems to to be the case
for instance on STM32 when no endpoint are being used.

Adding a 1 ms delay (i.e. one SOF period) between the detach and the
attach fixes the issue.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>