IB/core: fix error unwind in sysfs hw counters code

Between the initial and final versions of the function setup_hw_stats,
the order of variable initialization was changed.  However, the unwind
flow on error did not properly keep up with the flow changes.  Make
the unwind flow match a proper unwind of the allocation flow, then
remove no longer needed variable initializations.

Fixes: b40f4757 (IB/core: Make device counter infrastructure
dynamic)
Signed-off-by: Doug Ledford <dledford@redhat.com>