hw/arm/boot: fix uninitialized scalar variable warning reported by coverity

Coverity reports the 'size' may be used uninitialized, but that can't happen,
because the caller has checked "if (binfo->dtb_filename || binfo->get_dtb)"
before call 'load_dtb'.

Here we simply remove the 'if (binfo->get_dtb)' to satisfy coverity.

Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Message-id: 1416826240-12368-1-git-send-email-zhang.zhanghailiang@huawei.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
