iwlwifi: recalculate average tpt if not current

We currently have this check as a BUG_ON, which is being hit by people.
Previously it was an error with a recalculation if not current, return that
code.

The BUG_ON was introduced by:
commit 3110bef7
Author: Guy Cohen <guy.cohen@intel.com>
Date:   Tue Sep 9 10:54:54 2008 +0800

    iwlwifi: Added support for 3 antennas

... the portion adding the BUG_ON is reverted since we are encountering the error
and BUG_ON was created with assumption that error is not encountered.
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/iwlwifi/iwl-agn-rs.c

@@ -2077,10 +2077,12 @@ static void rs_rate_scale_perform(struct iwl_priv *priv,
 	}
 	/* Else we have enough samples; calculate estimate of
 	 * actual average throughput */
-
-	/* Sanity-check TPT calculations */
-	BUG_ON(window->average_tpt != ((window->success_ratio *
-			tbl->expected_tpt[index] + 64) / 128));
+	if (window->average_tpt != ((window->success_ratio *
+			tbl->expected_tpt[index] + 64) / 128)) {
+		IWL_ERR(priv, "expected_tpt should have been calculated by now\n");
+		window->average_tpt = ((window->success_ratio *
+					tbl->expected_tpt[index] + 64) / 128);
+	}
 
 	/* If we are searching for better modulation mode, check success. */
 	if (lq_sta->search_better_tbl &&