i2c: at91: Fix compilation warning.

Replace '%d' by '%zu' to fix the following type of compilation warnings:

drivers/i2c/busses/i2c-at91.c:277:2: warning: format ‘%d’ expects argument of type ‘int’, but argument 5 has type ‘size_t’ [-Wformat=]
  dev_dbg(dev->dev, "wrote 0x%x, to go %d\n", *dev->buf, dev->buf_len);
  ^

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Tested-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>