AHCP: Modernize packet parsing style.

AHCP: Modernize packet parsing style.

Similarly to earlier commits, follow up from commit 19b51ded and make
changes along the same lines. Also make all functions void, lose a few
excess ND_TCHECK_LEN() instances, update comments, reduce scope for a
couple variables, rearrange some line breaks and call ND_TCHECK_LEN()
for unknown protocol versions.