ichwd: add a few assertions about tco_version

Those should ensure correctness of ichwd_find_ich_lpc_bridge() and
ichwd_find_ich_lpc_bridge() as well as make it easier for both humans
and static analyzers to see the relation between tco_version and ich and
smb variables in ichwd_identify().

Reported by: Coverity
CID: 1396314, 1396317
MFC after: 10 days