drivers:dfu: dfu_flush(): add raw data flush to complete dfu write

drivers:dfu: dfu_flush(): add raw data flush to complete dfu write

Before dfu write and flush operations separation,
dfu write data was flushed by host download request
with len of zero size.

Since above change manually calling dfu write with zero
size has non sense (e.g. in THOR). This should be done by
flush operation.
So now dfu_write_buffer_drain() is called in dfu_flush().
If there is any raw data to flush (like it can be in thor)
then it will be physically written to medium.

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: Marek Vasut <marex@denx.de>