usb/class/usb_dfu: signal dfuMANIFEST_SYNCH after flashing is done

End of DFU operation was signaled after the last usb dfu request was
received. Therefeore wait_for_usb_dfu() terminated so fast that
dfu_work_handler() was not called by k_work_queue befor.

wait_for_usb_dfu() should terminate after DFU operation was completed.

For fix that k_poll_signal_raise() was moved after the flash write
operation.

fixes #29611

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>