firewire: add warn-only CRC validation for CSR ROM directories

Implemented crom_crc_valid() helper to validate IEEE 1394 config ROM CRC-16
checksums.

Skipped root header CRC validation since csrhdr.crc_len cover the entire
ROM body which is not fully read at header parse time. Per-directory
CRC checks below catch corruption where it needed.

Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D58307