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

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 <[email protected]>
Cc: Lukasz Majewski <[email protected]>
Cc: Marek Vasut <[email protected]>
Cc: Heiko Schocher <[email protected]>
Cc: Tom Rini <[email protected]>