libfdt: fix fdt_check_node_offset_ w/ VALID_INPUT

fdt_check_node_offset_() checks for a valid offset but also changes the
offset by calling fdt_next_tag(). Hence, do not skip this function if
ASSUME_VALID_INPUT is set but only omit the initial offset check in that
case.

As this function works very similar to fdt_check_prop_offset_(), do the
offset check there as well depending on ASSUME_VALID_INPUT.
Message-Id: <1913141.TlUzK5foHS@noys4>

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>