usb, dfu: extract flush code into seperate function

move the flushing code into an extra function dfu_flush(),
so it can be used from other code.

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
diff --git a/include/dfu.h b/include/dfu.h
index f973426..272a245 100644
--- a/include/dfu.h
+++ b/include/dfu.h
@@ -138,6 +138,7 @@
 
 int dfu_read(struct dfu_entity *de, void *buf, int size, int blk_seq_num);
 int dfu_write(struct dfu_entity *de, void *buf, int size, int blk_seq_num);
+int dfu_flush(struct dfu_entity *de, void *buf, int size, int blk_seq_num);
 /* Device specific */
 #ifdef CONFIG_DFU_MMC
 extern int dfu_fill_entity_mmc(struct dfu_entity *dfu, char *s);