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>