IB/core: Fix array length allocation

The new sysfs hw_counters code had an off by one in its array allocation
length.  Fix that and the comment along with it.

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