IB/mlx4: Fix mlx4_ib_add() error flow

We need to use a different loop index for mlx4_counter_alloc() and for
device_create_file() iterations: the mlx4_counter_alloc() loop index
is used in the error flow to free counters.

If the same loop index is used for device_create_file() and, say, the
device_create_file() loop fails on the first iteration, the allocated
counters will not be freed.

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <roland@purestorage.com>