[util/vendor_hw.py] Fix only_subdir support to allow patching.
[util/vendor_hw.py] Fix only_subdir support to allow patching.

Previous commit "[util/vendor_hw.py] Add support for 'only_subdir' key
in upstream section." left patching for only_subdir upstreams broken.
This commit fixes that oversight.

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan