ahc(4): resolve some minor nits

In ahc_init(), qoutfifo is already assigned to effectively the same
value a couple lines up, except in the first assignment it uses the
proper definition; keep the more descriptive assignment.

ahc_targetcmd_offset() gets the offset wrong entirely; as per the
area of ahc_init() this diff also touches, targetcmds is laid out first
in the shared map and it's followed by the qoutfifo. As a result, we'd
generally be getting negative offsets here. We can't actually do a
partial sync anyways, so there was no consequence to getting this wrong.

Reviewed by: imp, mav
Differential Revision: https://reviews.freebsd.org/D44859