x86: Correct error return value in mrccache_get_region()

This function doesn't use uclass_find_first_device() correctly. Add a
check that the device is found so we don't try to read properties from a
NULL device.

The fixes booting on minnoxmax.

Fixes: 87f1084a ("x86: Adjust mrccache_get_region() to use livetree")

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>