dfu: tftp: Fix arm64 build warnings

Fix two build warnings when building for arm64:

drivers/dfu/dfu_tftp.c: In function ‘dfu_tftp_write’:
drivers/dfu/dfu_tftp.c:59:37: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
  ret = dfu_write_from_mem_addr(dfu, (void *)addr, len);
                                     ^
and

drivers/dfu/dfu_tftp.c: In function ‘dfu_tftp_write’:
drivers/dfu/dfu_tftp.c:41:8: warning: format ‘%u’ expects argument of type ‘unsigned int’, but argument 4 has type ‘__kernel_size_t {aka long unsigned int}’ [-Wformat=]
  debug("%s: image name: %s strlen: %u\n", __func__, sb, strlen(sb));
        ^

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Lukasz Majewski <lukma@denx.de>