smbios: Do not look up children of invalid nodes

smbios: Do not look up children of invalid nodes

If there is no UCLASS_SYSINFO device available, parent_node will be
ofnode_null(). Calling ofnode_find_subnode() then triggers an assertion:

  drivers/core/ofnode.c:598: ofnode_find_subnode: Assertion `ofnode_valid(node)' failed.

Check for a valid parent_node, not just that OF_CONTROL is enabled.

Fixes: 44ffb6f0ecaf ("smbios: Allow properties to come from the device tree")
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>