Make the hardware counters available on BCM5705 and newer available as

Make the hardware counters available on BCM5705 and newer available as
kstats.  Some of the hardware counters are already used in
bge_stats_update_regs() to update interface counters and work around
hardware bugs, and all counters reset on read, so to keep things simple
the work is split between that function and bge_kstat_read(), which
reads the rest of the counters.

tested by bluhm@ on BCM5720 (with counters) and BCM5704 (without), and
by me on BCM5720 A0 (with counters and hardware bugs)
ok bluhm@ dlg@