usb: dfu: correct dfu buffer inited value

After dfu buffer is initialized, the buffer should be all available,
while not 0. Initialize its value to min(dfu_buf_size, dfu->r_left).

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>

drivers/dfu/dfu.c

@@ -288,7 +288,7 @@ int dfu_read(struct dfu_entity *dfu, void *buf, int size, int blk_seq_num)
 		dfu->offset = 0;
 		dfu->i_buf_end = dfu_get_buf() + dfu_buf_size;
 		dfu->i_buf = dfu->i_buf_start;
-		dfu->b_left = 0;
+		dfu->b_left = min(dfu_buf_size, dfu->r_left);
 
 		dfu->bad_skip = 0;