cam: Fix wiring fence post error

If the last matching device entry partially matched in camperiphunit,
but then hit a continue case, we'd mistakenly think we had a match on
that entry. This lead to a number of problems downstream (usually a
belief that we had a duplicate wiring hint because unit = 0 is the
default). Fix this by using a for loop that does the assignment before
the loop termination test.

Sponsored by: Netflix
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D33873