iwlwifi: cleanup iwl_good_ack_health

Make ack health code easies to read. Compared to previous
code, we do not print debug messages when expected_ack_cnt_delta == 0
and also do check against negative deltas.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>