usb:gadget:f_thor: fix write to filesystem by add dfu_flush()

Since dfu read/write operations needs to be flushed manually,
writing to filesystem on MMC by thor was broken. MMC raw write
actually is working fine because current dfu_flush() function
writes filesystem only. This commit adds dfu_flush() to f_thor
and now filesystem write is working.

This change was tested on Trats2 board.

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