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>